How can 'last-saved' values be easily cleared in Collect?

Agreed! I'd use it in particular when iterating on early drafts of a form definition. It'd be great to carry the same draft data over to new versions of the form definition even when the form definition could have changed quite a bit.

There's an Ideas thread about the general concept.

Yes, that's the idea. Probably available from the overflow menu (⋮) or something like that.