if you see a community for the first time, it will take a while to load the last 20 posts on that community. after that, every time an update is done on the remote community, it will be pushed to yours.

idk if at least one person need to subscribe to it, or just opening it is sufficient.

add multi community aggregation to aggregate communities of the same topic on different instances.

there is an app for mobile. that also offer continuous backup of photos (like gphoto)

if they have the same hash the deduplication thing will work. if they are different quality or other stuff no. thare are plans to implement something regarding more advanced deduplication but not anything implemented at the moment.

i use it with tree style tabs and it always worked. I don’t think there is an alternative that also support tree style tabs.

immich!

no idea, you should search issues on github.

Anyway comments are replicated across all instances, so nobody stops someone from running a modified instance that does not abide to the comment deletion request. Same for posts, idk about images.

you should consider everything that get published public forever.

there is comment edit history, even if you edit them the original will still be visible

report the posts, the moderators will ban the profiles.