Everything you do on your own instance is against a cached version of the original post that is saved on your instance. Your instance sends updates in the background, the other instance can be entirely down and you can still browse, comment, and vote as normal on your own. The updates will just stay local though.
It doesn’t. Even if you defederate with an instance, you can still comment and vote on posts from that instance that predate the defederation and people on your instance will see them, it just won’t go to the original one.
If you use Obsidian for work you generally need to pay for the commercial license, with some exceptions. I like to mention it because people grab it from flathub without reading the license terms. This is not including the sync fee.
It is not compatible with Kindle unless you remove the drm and convert it, but it can be done. It is compatible with Adobe Digital Editions which is pretty much what every store except Amazon uses.