I recommend getting familiar with torrenting, media servers, media players, and basic privacy stuff before using Seanime. Wiki. It does not provide any security guarantees around torrenting or streaming so you should use these features with a VPN.
AniList is the "database" that Seanime uses for user, anime and manga data. Its API is way more powerful and flexible than other alternatives such as MyAnimeList, and it allows Seanime to just build on top of it without having to maintain a tracking system of its own. If you're not using AniList, you should consider using an extension like MAL-Sync to keep your lists in sync between multiple services.
This is highly unlikely as most of the features are built on top of the AniList API in such a way that is almost impossible to mimic using other APIs. For example, something as complex as fetching the user’s entire anime collection which includes list entry data, relations and next airing episode for every single anime—in a single query.
No. You can use an extension like MAL-Sync to keep your lists in sync.
Docker support was dropped due to the complexity of containerizing it with all the dependencies needed for the various features. You can still try to containerize it yourself and contribute to the project.
You can use a VPN to bypass the block or use a different torrent provider (i.e. AnimeTosho).
You can run multiple instances of Seanime by specifying a different data directory. How to.
Technically yes, but, unlike other media servers, Seanime is built around the idea of a single user for a single instance. You can run multiple instances of Seanime on different ports for multiple different accounts and point them to the same media directory.