Hi, @stromfeldt. This is the kind of work that I do.
There are a lot of "this"s here, so I’m not quite sure how to answer.
I suspect that what you want is a bunch of instances on a single droplet in a multisite configuration. That’s a fairly complicated thing to get set up.
Next, I think, you want to configure one Discourse as the SSO provider and the rest as SSO consumers. There is no plug-and-play solution for that. An easier solution (that you could probably accomplish yourself) would be to use a Wordpress instance as the SSO provider.
Unless the Discourse WP plugin has some magic I don’t know about, I don’t think that there is an easy way to import just the existing comments from those multiple blogs to Discourse. I’d recommend having Discourse handle just the future comments on those blogs.
In the solution that I sort-of propose, the blogs would have both the comments that were part of that blog and the comments that were added on Discourse. You’d configure things such that all future comments were handled by Discourse, so in the future all comments would be handled by Discourse.