That would be an interesting argument if there were a copy of the Septuagint that used "like a lion" - there isn't. There ARE, however, MT manuscripts that are NOT consistent with the "like a lion" argument (though it now opens up whether the correct translation is "dug" or "pierced" - it is not "like a lion").
It's really a pretty simple argument... One of three things happened:
1) A weird coincidence that ends up causing unnecessary strife.
2) The Jews changed Scripture to support a theological position.
3) The Christians changed Scripture to support a theological position.
#3 is by far the least likely - since it requires Christianity to not only be false... but to have been planned a hundred or more years prior to the "time of Christ" in anticipation of the argument... and change it under the noses of the rest of the Jews.
All that would be necessary to disprove this is to show a manuscript that predates the need for the change that shows "like a lion". There is none. (Oh, and it would be nice to come up with SOME explanation for the grammar as well - but that's secondary).
You will argue till the cows come home about the Aramaic word for ROCK yet you cannot produce one single piece of paper. In this instance the particular missing piece of paper is meaningless?
Tell ya what; if I accept your argument for the correctness of "pierced" based on the absence of evidence will you abandon your argument for the correctness of "ROCK" for the same reason?
(Or "cousin" for that matter).