Update - so now I can shift gears again, they all come in quite easily (even the smallest sprocket, I really only ever use that downhill but here for some time I noticed it was harder to get into) and the original problem is solved (back pedalling no longer makes the chain jump), however, now the pedal motion just doesn't seem 'smooth'. Especially in the smallest sprocket but also the next 2-3, only really the 4th smallest does it start to feel 'smooth'. And hereby I mean it feels like when I turn the pedals it feels like they are held up. It doesn't feel like one smooth clean motion but more of a rr rr rr rr. Any ideas on that?

I hope I didn't bend something or whatever, as I said in the post above, for a while I think the chain was off the cassette even totally stuck.

Edit: So I loosened the B screw quite a bit (it could be that I tightened it earlier when I was getting frustrated and just trying things) and now the third smallest sprocket feels good, the 2nd smallest so la la and the smallest still kind of bad. But I think I can give the bike a spin and see how it is actually pedalling properly.