I think the easiest way to achieve this is to create your user register/login system outside of nodebb and use session sharing plugin on each of the communities. When users login on your system they will be logged in on all the nodebbs.
Yeah, SSLs.com, which used to be cheapSSLs.com. In my opinion, you do get the same cert for a much lower price, so I didn't see any reason not to go with 'em.
We're looking to introduce SSL support as an add-on for our premium hosting packages (bundled with village & city, extra for hamlets)