I don't think the rule is as black/white as you think. Personal experience is that most view this as a change in actuarial firms. The instructions state "Complete Part III if there was a termination in the appointment of an accountant or enrolled actuary". An "accountant" has never been interpreted as an individual as accounting firms often change the entire audit team each year and I think the same argument can be made for the actuary. I have seen some firms report internal changes, but I don't think it is very common.
1) No issue because the filing is not incorrect
2) No liability on your part. The service agreement is with the firm, not the individual actuary. Once you left the firm, you are no longer responsible.
3) If you retained the work at a different firm, I would show that as a change.
However, this brings up an interesting question for small plan land. TPAs that outsource actuarial work and hire independent people to review their work. I guess I feel a little differently if the TPA changes from basement actuary A to spare bedroom actuary B. That feels more like a change of actuary even though both might have signed using TPA's address and firm name.
I was assuming you left one actuarial firm for another, but if you are an independent signing stuff for a TPA, I might have a different opinion. Probably matters what your engagement letters say.