The mach you do not understand may be due to an NPE on your MIL's side but Occam's razor says that it is most likely the FIL side.
What you need to find out is where your MIL was 9 monhs before the birth of your wife and what was she doing work and leisure wise. How to cunningly ask that I don't know.
Then try to match that in with the relatives of your match.
Try contacting the match and see if they are willing to help. If they have gone to the trouble of making a tree then it is quite possible that they are sitting at home wondering why your wife should match with them.
Have you put your wife's tree online? Even with half of it missing it is better than nothing and shows that you are willing to share and not just stalking others.
PS - just had a look a Mick's earlier post
https://www.rootschat.com/forum/index.php?topic=401421.9The common possible ancestors are Alfred and Ethel who had 12 children so tracking where all the grandchildren were is going to be quite a slog.