"a. But we know memory eventually becomes static and unchanging--the story becomes set, so to speak. And we know all sensations dwindle to zero eventually. Nothing changes for the rest of time and since time is characterized by change, time has ended.
b. But eternity has no end so there must be some memory modification and sensations not at zero."

We don't know that memory becomes static and unchanging. This is a hypothesis that has not been tested. It's an assumption. We do know that memory is tied to the brain and is constantly changing as our perceptions of what we see in life change.

We don't even know physically what eternity is. If the universe(s) collapse, which is
theoretically a possibility, then what eternity is there? And if it's not there, why would it not have ended?

Once again the attempt to prove logically an argument is reliant on the agreement of premises and those you offer are unprovable by scientific means. They haven't been tested. They are assumptions.

The problem for the argument of a super "consciousness" that extends beyond the brain is not provable and one's experience of this supposed "consciousness" can only be supposed and not proven.