I have an 8 speed cassette on a wheel that's used mostly on my Diamondback Outlook which has a 7sp RD.
This wheel and cassette came with my 24-speed Sorrento when I bought it used. You can see the cluster has some rust on it but it's worked well for the past few months. I plan on keeping the wheelsets swappable, so I will probably get another 8 speed cassette unless you think that's part of my problem.
I have dialed the rear derailleur in so that it uses the outermost gears, never touching the granny gear on the cluster. A few days ago, I started to get slipping when in the 11t (smallest, highest) gear but only when I'm hammering.
Do you think it needs replaced? I'm assuming it's the original cassette that came with the Sorrento which would make it about 8 years old with unknown mileage. I have put about 400 miles on this cluster since I got it in September. I'm splitting my commute between two bikes depending on the weather and this wheelset mostly stays on my "Fair weather" bike.
Anyhow, on with the questions:
1) Does the 11t sprocket look worn out to you? I don't think it looks quite right.
2) The spacing of gears IS the same between an 8sp and a 7sp cassete, right? I didn't think there was a problem running this setup. I like being able to use either wheelset on either bike in a pinch.
3) The small sprocket is the lock washer on this cassette. Even though the rest of the cassette is ugly, I haven't had any problems with other gears. Could I just buy a new "11 tooth cassette lock washer" or would that be unwise?