User blog comment:SpaceManiac888/Series 9 Sweepstake/@comment-979907-20170301203642

Option 3 is that the unallocated robots go into a pool and when users lose their sweepstake in round one they immediately collect one from the pool. So if say Push to exit is eliminated but Behemoth progresses I get Behemoth, but if Behemoth goes I don't get another. This is still random allocation because users at this stage have an equal chance of getting a Heat A robot as a Heat E robot just like the unallocated have an equal chance of being spread through heads or clustered in one heat.

Alternatively you could ensure there's at least two unallocated per Heat which means taking eight robots and randomising six users during the allocation.

The more I think about it the more this makes sense as the best option