Bear,
I feel for you...yes your H is a pri@# and the OW is a who#@!!! I am sure that they both feel that they are doing nothing wrong. As far as you snooping is concerned, i know that it is not recommended but it is done and you should not regret it. let's face it you gathered some valuable information. It seems like you are determined and GO FOR IT GIRL! As far as what he has said, don't put too much weight on that. My W has said alot of things that she probably regrets today.
As far as your MIL is concerned, do what comes naturally. You are a good person and you should not let your H dictate what you feel you need to do as it pertains to his mom.
Stay strong and above all take good care of yourself in these difficult times. I just wish I could be more helpfull. H may some day realize what a P he has been and may even have a change of heart...it will be up to you to decide what to do if and when that happens. Until then, take care of the bear and surround yourself with people who love you.