Witnesses grant that Jesus is “a mighty god” and reserve “Almighty” for Jehovah. That Jehovah is also called mighty doesn’t stop a lesser being from being mighty too. Doesn’t this show the title is compatible with full deity, not that it requires it?

That is a fair statement of the logic, and precision helps. The argument removes the Witnesses’ claim that Mighty God is a title reserved for a lesser rank, since Isaiah applies it to Jehovah himself. It does not by itself prove the child is Jehovah. What it shows is that the two-tier reading has no support in Isaiah’s own usage, and that the child also receives the title Everlasting Father (or Father of eternity) in the same verse, which is harder to fit to a creature.