Hi, great questions!
1. Git pages allow hosting dynamic sites if the code works in the browser - most nostr apps are like that. Our engine can work on the server and in the browser, it connects directly to relays and converts events to html pages. Our hosting runs the code both on the server (to pre-render for search engines) and in the browser after the first page is loaded (to save our resources and to enable offline work). The self-hosted option we're offering is currently limited to browser-side rendering - so it can be hosted on Git pages, but can't be crawled by search engines properly. Server side engine coming later (won't work with Git pages as it will require nodejs).
2. Yes you can attach a custom (sub)domain with our hosting, and you definitely can add it if self-hosting.
3. Our alternative to your /blog/ collection would just be /posts/ (for long-form articles), and with static pages coming soon you'd be able to override the homepage and make other static pages like About me.
I suggest you just try npub.pro, and try importing a couple posts from your Ghost site and try playing with it. It should take 10 minutes to do all that and have something usable. You can always attach custom domain or self-host later, there's no "migration" - it's nostr, all the data is out there.
