Are "final decision" and "last decision" interchangeable?
Solution 1:
It honestly depends on context. If you were to say
"That was my last decision." vs
"That was my final decision."
you are likely to mean two different things, as you describe in your question. However, consider
"This will be my last decision." vs
"This will be my final decision."
In this context the former statement implies the latter, so the meaning of the two phrases is the same, whether you mean final in the sense of "not going to change" or final in the sense of "last in a series".