I'm trying to catch up on this discussion and this thread caught my attention :)
Having the user to 'open' the slot before the delegation will work from my point of view, but maybe worth considering also the following options:

1. each user can have max 3 delegations at a time
2. every time a new RC delegation is done we check if there is an empty slot for the receiver
   2a. if an empty slot is available just use it
   2b. if all slots are full the delegation with the lower amount is removed and the new one is accepted