OK, I've answered question 1 re: 7 domains for 4 sites.
The "co-domains" count as domains as well (one of the sites had no co-domain associated with it, I probably deleted it before finding out about the below paragraph's info).
My understanding is that co-domains are setup at site creation time as I've read in another thread the way you make the account creates a domain known as www.domain.xyz
(only place I've ever heard of this happening) and then create a co-domain of domain.xyz.
So, I've realised that I will need to remember to create accounts with at least twice as many "domains" as "sites", but the problem of this "You have left only 0 domain(s)" continues.