Hello,
1) For assembly code, right click somewhere in C file and select Disassemble.

2) If you use higher optimization, your code will be smaller, but also it will be more difficult to debug it and compiling will be slower. There is a description under each optimization level, so you can read, what optimizations are done.
3) It is possible that high level optimization will create some bug in the code. Unfortunately any compiler is flawless.
4) In case your code does not work as expected with optimization, I recommend you set optimization off and do not use it (if it is possible).
Hope it helps. In case you have any other questions, please feel free to write me back.
Regards,
Martin