prosites workaround - cross domain propagation

Topic discussed with Alex here:
https://premium.wpmudev.org/forums/topic/pro-sites-to-restrict-visitors-but-not-admins-sites?replies=8#post-510620

part two is:

I am seeking a work-around till this is resolved and it's mangling my head!

I thought: have domainhome and domainusers
Domainhome could host all MY sites (topic1.domainhome, topic2.domainhome etc.)
domianusers hosts the PROSITES stuff. (paidsite1.domainusers etc.)

Now I have control of my sites without prosites crippling my junior admins.

But.

How can I / Can I, enable a single site where people can sign up. If they join topic1.domainhome AND want a website, can they do this with a single account?

Can I get adverts and posts from ???.domainhome to propogate to *.domainusers?

I want a clean, single membership experience. Join and be a member - get just an account (no site), add posting (to domainhome, still no own site), add advertising, OR have all this AND own site.

I also need to pass info from ALL user accounts to mailchimp via Buddypress fields. I don't want people to be able to have a site UNLESS they are a member of ???.domainhome too.

So really I want people to be able to sign up from domainhome, but for the website to be created at domainusers. Is this possible?

This is at the limits of my tech knowhow. Does one of your 'domain' plugins enable me to sell 'across domains'? Any thoughts on how I should structure this?

Mike