If this is for animation work, a series of frames generated at different SL's may not provide a very good output. I'm guessing there will be a lot of jumping noise and artifacts. I'll try to answer your questions as best I can, but there may be better ways. If so, someone will chime in.
1) Short of opening up each MXI, I don't believe there is a quick way to check a series of individual SL's.
2) It sounds as if all of your renders reached a time limit and stopped as opposed to their required SL. In the future, set the time to something unreachable like 9999 minutes per frame, with an SL limit set to what you need (ie. 17). This way it'll reach the SL number, stop and move onto the next frame before the time runs out, ensuring all the frames reach the same level.
3) There may be a much better way of doing this, but I remember when I had 6 frames in an animation that I needed to re-render. I simply took those 6 frames, re-numbered them as "[animation name]001.mxs", "[animation name]002.mxs", "[animation name]003.mxs"... added them to the network as 1-6 and rendered them seperately. When they were done I renamed them back placed them back in with the others. I was lucky enough to only have 6 frames to re-do, however. If you had a few hundred it may not be a good solution.