That's a horrible situation. Unfortunately, by the time someone decides to leave a relationship, they've already checked out emotionally - often because they've got somebody else lined up, but very rarely admit it. (I've got the t-shirt). Very tough for the one left behind, you've had all this come as a shock, plus the worry about your daughter. Do you think your wife would lie about how the bruise came about? I don't know your daughter's age, or if she could be coerced into covering up how it happened.
It's early days, you must allow yourself to grieve for the relationship - you'll go through similar emotions as people do when adjusting to the passing of a loved one. I didn't really go through the "anger" phase, which made it hard to let go. Good advice from Nogbad re. talking to the mother-in-law, maybe sound out what she thinks.
Please don't let you mental health deteriorate, contact your GP if necessary. You may well have already done talking therapies for the PTSD - it's something which I've found helpful in the past.
Good luck - each day is a tiny step forward to a new beginning.