Is it still possible to get locked levels?

Solution 1:

I don't think so.

When I've seen the salvage jobs to Csilla, Endor, and Kashyyk, I've looked with the little 'magnifying glass tool' to see what the possible salvage rewards are and I haven't seen anything but various characters in a long time, at least several months. So I think either the possible salvage list tool is broken (I do remember it working a while ago, listing other possible rewards) or the other rewards are no longer available.

I never did see information on how likely the 25 bux missions were to get a new level, so it's hard to tell if your 8 attempts are enough to conclude that it isn't going to happen.