Andy,
Any Player at the table can ask to see a called hand, that includes the pot winner. Once a hand is in the muck, it is dead.
To answer the second part of your question; TDA rule #11 clearly indicates the proper order of showdown. If I understand you correctly, you are asking if all players check on the last round of betting, do we look back at the last aggressive action on a previous round? The answer is, no. There used to be some casinos that would actually go back to a prior betting round but, that has been changed and I doubt that any card rooms still use that rule. If the TDA rule isn't enough for you then it is the last betting round which, in your example would eliminate the SB on that round because he would have obviously checked before the BB bet.