Check the chain with a checker, if it is past due, may have killed the cassette and chainring. You will know if you put a new chain on and it starts skipping or grinding.
It's hard to judge a cassette as the teeth are not all uniform. Check the chain for stretch. A cassette will last quite a few chains before wearing out.
Lol... Why would they listen to you? Stop making sense damnit!If the b tension isn't right, the gap from the upper pulley and bottom of the 50t will be off and force the chain to sit just off of the teeth. When you pedal it will seat right in, but off the bike it will do what you see.