Need advice on Domain Access setup

I tried to switch an existing D7 installation (currently with only one site) to use Domain Access, and quickly realized there were several areas where I wasn't sure how to proceed.

First, to describe the existing setup... I have several D7 sites that share a single multi-site installation. All those sites are completely unrelated to each other, and each one should have its own DB, theme, etc. Currently, only one of those sites has been moved to a VPS host account, and that's the one I'm trying to switch to Domain Access. It's structure is...

sites/default <-- no site here yet
sites/mysite.com <-- existing site (a forum site with all uploaded files/images in sites/mysite.com/files)

The goal... That site is about to become just one of several related sites (about racing simulators) that should have, at the very least, a shared user table - that is, anyone who registers in one site should automatically be a member of all sites. One of the sites in that "family" of sites has two distinct versions that are related by name, but need to have separate forum content. It is the existing site described above, so it should be "split" (logically, not physically) into v1.mysite.com and v2.mysite.com. The main domain of those two should still act as a landing page for visitors to select which version they are interested in, and there will be some content that can be seen by users of both versions.

Also, since I am creating a "family" of related sites, I want to have an overall top-level landing page that shows icons that link to each of the racing sim sites. Let's call that one mymainsite.com.

So here's what I need...

mymainsite.com - main landing page for all racing sim sites
mysite.com - current existing site containing 15,000 forum posts for both versions
v1.mysite.com - new split of v1 from mysite.com
v2.mysite.com - new split of v2 from mysite.com
anothersite.com - another (future) racing sim site
yetanothersite.com - and so on...

So, first question is... the Domain Access "site" should be set up in /sites/default, correct? The issue there is moving all of the existing uploaded files from /sites/mysite.com/files to /sites/default/files. I guess the best way is to do a backup (with Backup/Migrate module), then edit the .sql file to change the paths and URLs. The difficult part of that is not the files per se, but the embedded links to other parts of the site. There are more than 4400 of them, some of which will need to change to v1.mysite.com and others to v2.mysite.com.

For persistent logins, should the $cookie_domain setting be set to ".mymainsite.com"?

And, finally, when I start moving my other unrelated D7 sites over, will it work for them to be set up in the normal Drupal multi-site fashion? By that, I mean if the Domain Access-enabled setup is in /sites/default, and all other sites are in /sites/site-specific-folder.com, each with their own DB credentials (and with Domain Access module NOT enabled), will that work?

Thanks in advance.