(didnt read the whole thing, yeah, I know) Its just seemed out of place to find a seal from a letter with the inscription Isaiah the prophet and attribute the seal to Isaiah’s time unless he was commonly known as that at that time.
Its still very early, here in my head.
Then Isaiah the prophet went to King Hezekiah and asked, What did those men say, and where did they come from?
From a distant land, Hezekiah replied. They came to me from Babylon. --Isaiah 39:3 (NIV)
I don't know if the Hebrew phrase for "Isaiah the prophet" was the same as the the Hebrew on the seal, but at least the translation to English in this article and from the NIV bible match.