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The virgin birth is not only a mistranslation but something they borrowed from the pagans.

Isaiah 7 (Hebrew Names Version)
14 Therefore the Lord himself will give you a sign: behold, an almah shall conceive, and bear a son, and shall call his name Immanu'el.

from the Hebrew
5959 `almah al-maw' feminine of 5958; a lass (as veiled or private):--damsel, maid, virgin.
1) virgin, young woman
a) of marriageable age
b) maid or newly married

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There is no instance where it can be proved that this word designates a young woman who is not a virgin. (TWOT)

However.... there is also no reason to disregard the other options for the use of the word almah, instead of the word bthuwlaw, which unlike the word used in Isaiah, has only one meaning. Virgin!

from the Hebrew
1330 bthuwlah beth-oo-law' feminine passive participle of an unused root meaning to separate; a virgin (from her privacy); sometimes (by continuation) a bride; also (figuratively) a city or state:--maid, virgin.
1) virgin

Had the author of Isaiah wanted to make sure that everyone knew the young woman was a technical virgin, he would have used the word that actually meant, a technical virgin, instead of a word that could be taken to mean, young woman, or lass.

Isaiah 7 (Good News Translation)
7:14Well then, the Lord himself will give you a sign: a young woman F11 who is pregnant will have a son and will name him "Immanuel.' F12 R14

F11: young woman: [The Hebrew word here translated "young woman" is not the specific term for "virgin," but refers to any young woman of marriageable age. The use of "virgin" in Mt 1.23 reflects a Greek translation of the Old Testament, made some 500 years after Isaiah.]

'parthenos' is not used exclusively for 'a virgin', the LXX verse below refers to Dinah as 'parthenos' after she was raped by Shechem in Gen 34:3.

Genesis 34:3(LXX Greek)

English Translation -- from the Greek 3933 virgin
Original Word -- parqevnoß
Transliterated Word -- parthenos

parthenos 1 from an online greek dictionary (you'll have to scroll down towards the bottom)
* I. a maid, maiden, virgin, girl, Hom., etc.
* 2. Parthenos, as a name of Athena at Athens, of Artemis, etc.
* II. as adj. maiden, virgin, chaste, parthenon psuchên echôn Eur.: metaph., p. pêgê Aesch.; parthenoi triêreis maiden, i. e. new, ships, Ar.
* III. as masc., parthenos, an unmarried man, NTest.
[deriv. uncertain] 1 parqe/nos, h(,

Not only that, but:

Justin Martyr admits that christianity offers nothing new, that the pagans and their pagan gods didn't already have.

And when we say also that the Word, who is the first-birth45 of God, was produced without sexual union, and that He, Jesus Christ, our Teacher, was crucified and died, and rose again, and ascended into heaven, we propound nothing different from what you believe regarding those whom you esteem sons of Jupiter. For you know how many sons your esteemed writers ascribed to Jupiter: Mercury, the interpreting word and teacher of all; Aesculapius, who, though he was a great physician, was struck by a thunderbolt, and so ascended to heaven; and Bacchus too, after he had been torn limb from limb; and Hercules, when he had committed himself to the flames to escape his toils; and the sons of Leda, and Dioscuri; and Perseus, son of Danae; and Bellerophon, who, though sprung from mortals, rose to heaven on the horse Pegasus.
But far be such a thought concerning the gods from every well-conditioned soul, as to believe that Jupiter himself, the governor and creator of all things, was both a parricide and the son of a parricide
Moreover, the Son of God called Jesus, even if only a man by ordinary generation, yet, on account of His wisdom, is worthy to be called the Son of God; for all writers call God the Father of men and gods. And if we assert that the Word of God was born of God in a peculiar manner, different from ordinary generation, let this, as said above, be no extraordinary thing to you, who say that Mercury is the angelic word of God. But if any one objects that He was crucified, in this also He is on a par with those reputed sons of Jupiter of yours, who suffered as we have now enumerated.
And if we even affirm that He was born of a virgin, accept this in common with what you accept of Perseus. And in that we say that He made whole the lame, the paralytic, and those born blind, we seem to say what is very similar to the deeds said to have been done by Aesculapius.

Justin Martyr gives a point, by point, by point, accounting of the similarities between paganism and the 'new religion'. Its the same thing under a new name.

You wrote:

“The virgin birth is not only a mistranslation but something they borrowed from the pagans.”

Actually there’s no reason to believe it’s either.

“Justin Martyr admits that christianity offers nothing new, that the pagans and their pagan gods didn’t already have.”

No. What Justin Martyr did was try to show the pagan emperor that Christianity could be understood by someone who was already familiar with many pagan stories. Some would say that similar points can be made about the Old Testament as well.

“Justin Martyr gives a point, by point, by point, accounting of the similarities between paganism and the ‘new religion’. Its the same thing under a new name.”

No. The incarnation of the God man so that He could die on a cross and redeem all mankind and rise from the dead, is uniquely Christian. Yes, pagan religions reflect many or even most of those ideas. But no pagan religion has all that Christianity has.