Christos,

... here's another complicating scenario not already mentioned as of
me writing this: When someone gets unsubscribed due to spam, if they
are a free-slot member, the code must maintain that slot available,
for a while at least, because we wouldn't want that member to click on
the resubscribe link only to be told, "sorry, you now have to pay".
But then, how long do we keep that slot reserved??

That's an interesting point.

Clearly the slot should stay reserved to that (former) member's address for at least the 7-day duration that the "Resubscription" link is active:
https://groups.io/helpcenter/membersmanual/1/working-with-group-messages/responding-to-a-you-have-been-removed

If the group owner does not want to keep the slot reserved that long then (under my suggested enhancement) the owner could locate the address in the Past Members list and "bump" that former member out of the free slot.

Except in Basic groups, which don't have the Past Members list. This might be sufficient reason to extend that feature to Basic groups (who are most in need of managing their limited number of slots).

Shal