Multisite Users within Wider Site Context

So I'm setting up a website, let's call it hobbeez.com (that's not it but you know, non disclosure blah blah). Users will create accounts on hobbeez.com and do what they do. Running along side this is a blog farm at hobbeez.me which is naturally built on multisite

Now, looking at user integration. This is where the head butting is starting.

When a user creates an account at hobbeez.com the idea is that they would automagically get an account set up at the blog farm. Their avatars should flow through and username/password issues should all be sorted via the main site. So far so good.

What if an outsider wanted to comment on a blog, or to become a member of it, do they need to become a member of the entire hobbeez.com website? What's the best way to do that? Ideally we wouldn't take them away from the blog they are joining

If Mary's dad wants to comment on her blog post I'm sure that he won't be wanting to be forced into a full hobbeez.com account in order to do so.

If they don't become a member of the full website, what happens when someone wants to join the full website and wants the same username as that first user has on the blog site?

If we say that the two are separate and never will meet in terms of user integration, what if the user forgets their password on one? They could conceivably end up with multiples which would be annoying - or do you think it would be OK to keep them separate and trust that people can work it out?

Has anyone got any thoughts? I know that my brain is mush and seems to just start oozing whenever I try to think of how to do this properly.