Leavened vs Unleavened bread, Does artos always mean leavened bread?

On the issue of leaven bread vs unleavened bread, some claim artos always means leavened bread and cannot mean unleavened bread, and since Christ used artos in the last supper, therefore leavened bread was used.

Jesus refers to the showbread simply as artous:

how he entered the house of God and took and ate the bread (ἄρτους) of the Presence, which is not lawful for any but the priests to eat, and also gave it to those with him?”--Luke 6:4

Ὡς εἰσῆλθεν εἰς τὸν οἶκον τοῦ θεοῦ, καὶ τοὺς ἄρτους τῆς προθέσεως ἔλαβεν, καὶ ἔφαγεν, καὶ ἔδωκεν καὶ τοῖς μετ' αὐτοῦ, οὓς οὐκ ἔξεστιν φαγεῖν εἰ μὴ μόνους τοὺς ἱερεῖς;--Luke 6:4

The bread is found in the book of Exodus and called lehem (bread) in Hebrew, artous in Greek:

 And thou shalt set upon the table showbread (לֶחֶם פָּנִים) before Me alway--Exodus 25:30

וְנָתַתָּ עַל-הַשֻּׁלְחָן לֶחֶם פָּנִים, לְפָנַי תָּמִיד.--Exodus 25:30 MT

 And you shall place upon the table facing loaves (ἄρτους), in front me always.--Exodus 25:30 NETS translation of LXX

καὶ ἐπιθήσεις ἐπὶ τὴν τράπεζαν ἄρτους ἐνωπίους ἐναντίον μου διὰ παντός--Exodus 25:29 (30) LXX

The Bread of the Presence is referred to by Josephus as both artous (bread) and azymous (unleavened).

Upon this table, which was placed on the north side of the temple, not far from the most holy place, were laid twelve unleavened loaves of bread, six upon each heap, one above another: they were made of two tenth-deals of the purest flour, which tenth-deal [an omer] is a measure of the Hebrews, containing seven Athenian cotyloe; and above those loaves were put two vials full of frankincense. Now after seven days other loaves were brought in their stead, on the day which is by us called the Sabbath; for we call the seventh day the Sabbath. But for the occasion of this intention of placing loaves here, we will speak to it in another place.--Josephus Antiquities of the Jews, Book 3.142 

ἐπὶ ταύτης, ἐτίθετο γὰρ ἐν τῷ ναῷ τετραμμένη πρὸς ἄρκτον οὐ πόρρω τοῦ μυχοῦ, διετίθεσαν ἄρτους τε δώδεκα ἀζύμους κατὰ ἓξ ἐπαλλήλους καθαροῦ πάνυ τοῦ ἀλεύρου ἐκ δύο ἀσσάρων, ὃ μέτρον Ἑβραίων ἑπτὰ κοτύλας Ἀττικὰς ἔχει. --Josephus Antiquities of the Jews Book 3.142

Continuing on the same subject, Josephus simply referred to it as simply arton/artoi

and above those loaves (ἄρτων) were put two vials full of frankincense. Now after seven days other loaves (ἄρτοι) were brought in their stead, on the day which is by us called the Sabbath; for we call the seventh day the Sabbath. But for the occasion of this intention of placing loaves here, we will speak to it in another place. 

ὑπὲρ δὲ τῶν ἄρτων ἐτίθεντο φιάλαι δύο χρύσεαι λιβάνου πλήρεις, μετὰ δὲ ἡμέρας ἑπτὰ πάλιν ἄλλοι ἐκομίζοντο ἄρτοι ἐν τῷ καλουμένῳ ὑφ᾽ ἡμῶν Σαββάτῳ: τὴν γὰρ ἑβδόμην ἡμέραν Σάββατα καλοῦμεν: τὴν δ᾽ αἰτίαν ἐξ ἧς ταῦτα ἐπενοήθησαν ἐν ἑτέροις ἐροῦμεν.  

 Josephus saw no issue in calling unleavened bread as ἄρτους. This undermines the claim artos always means leavened bread.

In addition, Jews regard the bread as matzah, unleavened bread like that used for passover.

This Jewish article states the temple show bread was unleavened:

It was called bread, but it was matzah. It stood in the open air for a full week and didn’t become stale. And its preparation required special expertise.--The Showbread: The How and Why of the Temple Bread Offering

The Talmud says:

it is written here bread’,14 and it is also written in connection with the Two Loaves ‘bread’;15 as there there was one tenth [of an ephah] for each loaf, so here there must be one tenth for each cake. Or perhaps argue thus: it is written here ‘bread’, and also there in connection with the Shewbread it is written ‘bread’;16 as there there were two tenths for each loaf, so here there must be two tenths for each cake. Let us then see to which of the two is this case most similar. We may infer a meal-offering which is leavened and offered with an animal-offering17 from another meal-offering which is leavened and is offered with an animal-offering,18 but let not the Shewbread enter into the argument seeing that it is neither leavened nor offered with an animal-offering.--Talmud, Tractate: Menachoth 77b 

In a long debate about over Leviticus 2:11 and when and how the prohibition of leaven is applied, one rabbi mentioned:  

R. Akiba says, It includes the Shewbread, so that it too comes within the prohibition of leavening.--Talmud, Tractate: Menachoth 57b 

As a side note, the Talmud says also:

they used to lift it [shewbread table] up and exhibit the Shewbread on it to those who came up for the festivals, saying to them, Behold, God's love for you!--Menachoth 29a

Reminiscent of "behold the lamb of God" when the eucharistic host is lifted up by the priest in the Roman rite.

One reason for why it's unlikely leavened bread was used for the showbread was because Leviticus banned its use:

No grain offering that you bring to the LORD shall be made with leaven (חָמֵץ), for you shall burn no leaven nor any honey as a food offering to the LORD.--Leviticus 2:11

כָּל-הַמִּנְחָה, אֲשֶׁר תַּקְרִיבוּ לַיהוָה--לֹא תֵעָשֶׂה, חָמֵץ:  כִּי כָל-שְׂאֹר וְכָל-דְּבַשׁ, לֹא-תַקְטִירוּ מִמֶּנּוּ אִשֶּׁה לַיהוָה.--Leviticus 2:11 MT 

You shall not prepare any sacrifice that you may bring to the Lord, with leaven (ζυμωτόν), for you shall not present any leaven (ζύμην) and any honey to offer of it to the Lord.--Leviticus 2:11 (NETS translation of LXX)

πᾶσαν θυσίαν ἣν ἂν προσφέρητε κυρίῳ οὐ ποιήσετε ζυμωτόν πᾶσαν γὰρ ζύμην καὶ πᾶν μέλι οὐ προσοίσετε ἀπ᾽ αὐτοῦ καρπῶσαι κυρίῳ --Leviticus 2:11 LXX

For this reason, it seems unlikely the Eucharist would use leavened bread. 

Cleanse out the old leaven (ζύμην) that you may be a new lump, as you really are unleavened (ἄζυμοι). For Christ, our Passover lamb, has been sacrificed.--1 Corinthians 5:7

Ἐκκαθάρατε τὴν παλαιὰν ζύμην, ἵνα ἦτε νέον φύραμα, καθώς ἐστε ἄζυμοι. Καὶ γὰρ τὸ Πάσχα ἡμῶν ὑπὲρ ἡμῶν ἐτύθη χριστός·--1 Corinthians 5:7 Byzantine Majority

St Paul says to cast out the leaven and be unleavened, and that Christ is our passover. For the passover, leaven was strictly forbidden.

Seven days shall ye eat unleavened bread; howbeit the first day ye shall put away leaven out of your houses; for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from Israel. --Exodus 12:15

שִׁבְעַת יָמִים, מַצּוֹת תֹּאכֵלוּ--אַךְ בַּיּוֹם הָרִאשׁוֹן, תַּשְׁבִּיתוּ שְּׂאֹר מִבָּתֵּיכֶם:  כִּי כָּל-אֹכֵל חָמֵץ, וְנִכְרְתָה הַנֶּפֶשׁ הַהִוא מִיִּשְׂרָאֵל--מִיּוֹם הָרִאשֹׁן, עַד-יוֹם הַשְּׁבִעִי.--Exodus 12:15

The Greek translation of the LXX reads:

For seven days you shall eat unleavened (ἄζυμα) bread, but from the first day you shall remove leaven (ζύμην) from your houses. Everyone, whoever eats leaven (ζύμην), that soul shall be destroyed from Israel, from the first day until the seventh day.--Exodus 12:15 (NETS translation of LXX)

ἑπτὰ ἡμέρας ἄζυμα ἔδεσθε ἀπὸ δὲ τῆς ἡμέρας τῆς πρώτης ἀφανιεῖτε ζύμην ἐκ τῶν οἰκιῶν ὑμῶν πᾶς ὃς ἂν φάγῃ ζύμην ἐξολεθρευθήσεται ἡ ψυχὴ ἐκείνη ἐξ Ισραηλ ἀπὸ τῆς ἡμέρας τῆς πρώτης ἕως τῆς ἡμέρας τῆς ἑβδόμης--Exodus 12:15 LXX

The main argument that Christ used leavened is based on the assumption that the Last Supper was not a Passover meal, regardless, Paul certainly believed Christ was the Passover sacrifice.

We see St Cyril of Jerusalem believed the showbread was a foreshadow of the eucharist:

5. In the Old Testament also there was show-bread; but this, as it belonged to the Old Testament, has come to an end; but in the New Testament there is Bread of heaven, and a Cup of salvation, sanctifying soul and body; for as the Bread corresponds to our body, so is the Word appropriate to our soul.--St Cyril of Jerusalem. Catechetical Lectures: chapter 22

Likewise, St Basil of Caesarea:

He proclaims the renewal of the Temple, the observance of the worship of the Law, a typical high priest over again after the real High Priest, and a sacrifice for sins after the Lamb of God Who takes away the sin of the world. [John 1:29] He preaches partial baptisms after the one baptism, and the ashes of a heifer sprinkling the Church which, through its faith in Christ, has not spot or wrinkle, or any such thing; [Ephesians 5:27] cleansing of leprosy after the painless state of the resurrection; an offering of jealousy when they neither marry nor are given in marriage; show-bread after the Bread from heaven; burning lamps after the true Light. In a word, if the law of the Commandments has been done away with by dogmas, it is plain that under these circumstances the dogmas of Christ will be nullified by the injunctions of the law. --St Basil, Letter 265

St John of Damascus when writing about the eucharist states:

With bread and wine Melchisedek, the priest of the most high God, received Abraham on his return from the slaughter of the Gentiles. [Genesis 14:18] That table pre-imaged this mystical table, just as that priest was a type and image of Christ, the true high-priest. [Leviticus xiv] For you are a priest for ever after the order of Melchisedek. Of this bread the show-bread was an image. This surely is that pure and bloodless sacrifice which the Lord through the prophet said is offered to Him from the rising to the setting of the sun [Malachi 1:11] . --St John Damascene, An Exposition of the Orthodox Faith, Book IV: chapter 13

Likewise, when discussing the traditional practice of abstinence from sex prior to receiving communion (along the same lines Roman priestly celibacy is linked to this too), St Jerome, St Augustine, and St Gregory the Great cite the passage in the gospel about the show bread:

I am out of order in violating historical sequence, but I may point out that the same thing was said by [1 Samuel 21:4] Ahimelech the priest to David when he fled to Nob: "If only the young men have kept themselves from women." And David answered, "of a truth about these three days." For the show-bread, like the body of Christ, might not be eaten by those who rose from the marriage bed.--St Jerome, Against Jovinianus (Book I)

Hence also it is said through the priest to David concerning his servants, that if they were pure from women they might eat the show bread; which they might not receive at all unless David first declared them to be pure from women. Still a man who after intercourse with his wife has been washed with water may receive even the mystery of sacred communion, since according to the opinion above expressed it was allowable for him to enter the church.--Pope Gregory the Great, Registrum Epistolarum: Book XI, Letter 64 

The argument that leavened bread should be used for the eucharist mostly only works within the framework of Byzantinized Christianity that sees the leaven as symbolic of life and resurrection. The Biblical symbolism largely favors the unleavened bread.