Fairly sure its before the actual sign in takes place. This "path? access rights?)" is a local issue.

No, accounts cannot be deleted at WCG, but you could 'detach' WCG from the client and re-attach.... that would refresh a few local references.

Using different member names on different computers has no conflict, not even if the email account is identical. That latter will result in different members merging the points at e.g BOINCstats.