Specifically if the runner accesses archives and there is a single agenda in there can they use imp to avoid scoring it?

I'm pretty sure you can't. Trashing cards just means putting them onto the discard pile, which you can't really do if it's already there. Besides, I don't think anyone would want to figure out what to do if a card entered a server mid-access.

Cards already in archives cannot be trashed.

Also, I don't think Imp would prevent you from stealing an Agenda on access as it gets stolen at that point before trash options are considered (but if there were additional costs they could be left unpaid and then the agenda trashed using Imp).

Cards in archives can't be trashed.

However if accessing from R&D, HQ, or a remote, you can indeed use Imp to trash an agenda.

Actually yes on agenda trashing, it is under demolition run in the FAQ, my bad.