It's practically impossible to gauge what mods will work - it depends entirely on the background code that's updated/changed/removed when the devs finally release Mistlands. Mods tend to be a reactive resource as opposed to a proactive. There are a few bigger, well-known ones like Valheim Plus which are updated within 24 hours of patches and new releases.

It also heavily depends on your mod.
It's highly unlikely the devs would change existing systems like how damage is calculated much, and stuff like UI improvements tend to be heavily advertised because that's something easy to show and amaze with.
Really I would expect something along the lines of the ice cave update, where mods will generally still work the same but won't have the new stuff included yet (which could need to new issues either way though).

The most likely thing that's gonna happen is that BepInEx is gonna give out, which in turn will shut a lot of things down, it's gonna get fixed withing a a few days and then most things will probably work again with the old stuff, with new things depending on how quickly it gets implemented by the modders.

The best guess is that none will work immediately, and most will work eventually.

It's really hard to say, it depends on what has changed in the game code. During active development, all kinds of tinkering can go on (improvements, cleaning up messy code, moving functionality around, renaming methods or variables, ...), even if the code isn't part of the new content. I don't think predictions are possible, but your chances are better the more active a mod author is. After the next Valheim update, they'll need time to figure out the changes and update their own code.