They stay in the accounts of the banned person. I remember something similar happening in the past, and the person was advised to talk to the Admin to see if they could get their credits/assets back, since the banning had prevented completion of the deal.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Assets do indeed remain in the banned account, since bans are not necessarily permanent.
But, if a banned multi is unbanned, they do suffer partial asset loss, meaning that some of their assets are deleted upon their return.
Generally assets are not returned to players who sent something to the banned individual since multis often use their illegal multi to steal, and thefts are not undone. You might be able to plead your case with firstname.lastname@example.org but you should not get your hopes up.
Yes, there are. First of all, when a multi is unbanned they may only have one account unbanned, to ensure that they will not be able to violate the rules in the future. So if you and your friend are sharing a connection but the two of you start sharing accounts and are banned for it, only one may be unbanned - the other remains banned permanently. Second, if an illegal multi has one account unbanned and violates the rules a second time, they receive a permanent ban for blowing their second chance. Third, occasionally a multi ban is deemed sufficiently severe to warrant a permanent ban, but that is rare.. a permanent ban is more common if another rule is broken in addition, such as exploitation of a feature, hacking, or other rules violations. Multi bans used to be essentially permanent, but that was since replaced with the policy of unbanning one multi with partial asset loss to give players a second chance.
If a banned player is level 1, the normal inactivity rule applies where they are deleted after 9 months of inactivity. As far as I know, their assets are not deleted, though, but the death rules take precedence instead, so commanded/piloted entities go to their commander/pilot, unassigned entities to the market.