It is a valid argument against proof texts that are used to promote an extra-Biblical doctrine.
If the extra-Biblical doctrine is different from what was believed by the first century Christians, then the proof texts used to support that change in belief must be examined with extra care to find out if there were any changes made.
When John translated the verse, it said "look upon him" and not "look upon me".
Sorry, failed argument. Not only do you lack any document of any sort to support that allegation, but even a comparison of John's other quote concerning the broken bones indicates that John is applying the text, and making slight adjustment for grammatical usage. Or does your argument also shift that the translators corrupted John on this quote as well?
Psalms 34:20 KJV
(20)
He keepeth all his bones: not one of them is broken.
John 19:36 KJV
(36) For these things were done, that the scripture should be fulfilled,
A bone of him shall not be broken.
The fact is that John attributed the passage in Zechariah as relating to Jesus which answers your theory that the "first century Christians did not perceive Jesus as God." You cannot get more first-century Christian than John and Paul. John clearly understood Jesus as God.
Zechariah 12:10 KJV(10) And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications:
and they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn.
John 19:37 KJV
(37) And again another scripture saith,
They shall look on him whom they pierced.
Even if you claim the gospel of John was corrupted, what of Revelation here? Or do you just suggest that the writer John was corrupted?
Revelation 1:7-8 KJV
(7)
Behold, he cometh with clouds; and every eye shall see him, and they also which pierced him: and all kindreds of the earth shall wail because of him. Even so, Amen.
(8) I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty.
That conspiracy theory does not hold up under scrutiny. It reminds me of the Jehovah's Witness argument to justify why they placed "JEHOVAH" left and right in the New Testament. When pressed I was shown the translation foreword, which admitted that "Jehovah" appeared in no known New Testament manuscript, but that they placed it there because "we all know it should have been there because no Christian would have written the New Testament without it!" At this point I throw up my hands and ask them, "Seriously?"
Essentially you are doing the same thing. Each and every place that attests to the identity of Jesus as Lord and God you are willing to strike out under the rationalization that it "must have been corrupted.... " by someone or somehow or someplace. With that foundation, I don't know why you think you can trust anything in the scripture at all, because that same argument could be used for anything, it needs no evidence.