You can assume whatever you wish. The Tanakh was written in Hebrew. Let's assume for a moment that the oldest Greek text is older than the oldest Hebrew text. The fact remains that it still was a translation from the Hebrew into the Greek. For your argument to hold water, you also have to assume that somehow the translation preserves the accurate meaning, while the Hebrew has become corrupted. But, if you assume the Hebrew has become corrupted, you have no reason to believe that this did not happen prior to the translation into Greek as well. If we cannot have confidence in the accuracy of the original, why should we have confidence in the accuracy of the translation? It is worth noting as well exactly how much importance the scribes put upon accuracy when creating new scrolls.
It is also to assume that God had a hand in the writing of the Hebrew text but not that of the Greek,
There are those who believe that the KJV is an inspired translation, too. Did God have a hand in the writing of the KJV? What evidence can you provide that the Septuagint was a divinely guided translation? Furthermore, to suggest that God guided the translators to create a meaning that was different than that found in the original is to suggest that God made a mistake in the original that needed to be corrected in the translation. Sorry, but I find this absurd.
The SOLE reason you favor the Greek translation over the Hebrew is precisely because the errors and ambiguities of translation conveniently support your theological leanings. If this is okay, then the Proddies can translate the scriptures in such a way as to lend greater support to their beliefs, and you have no grounds for criticism of them for doing so.
which is to say that revelation was "sealed" when the last Hebrew work was written.
Yes, that is what I believe.
This charge can be made against your position as well. You assume the purity of the Hebrew text, but why? because those who have established your canon believe this.
If this is okay, then the Proddies can translate the scriptures in such a way as to lend greater support to their beliefs, and you have no grounds for criticism of them for doing so.
No,the comparison is wrong. We take as Scripture writings that your canon does not. Our claim is more like that of the Muslims, who believe in a later , FULL, and final revelation. To them both Jewish and Christians Scriptures are sacred, but the Koran is the PERFECT word of God.