Hi there!
1. I recognize this is probably the biggest technical challenge - it would be nice if possible, but I can imagine it's likely not possible unless you already have a backup.
2. In an ideal world it would be nice if this was possible, but even a migration without those things (like GTS and Sharkey can do) would be a massive step up from what Mastodon currently does.
3. This would make the most sense as a user option to pick when performing the import, at least in my opinion.