Lou and kml both have given you great advice. I would be civil to the MIL, but I would put some distance between you and her. You don't know what she's up to and it sounds like she was playing nice to soften you up for the divorce process.
As for going to the gravesite...I wouldn't do it. You don't know how your h is going to behave around you and you don't want to be ambushed by them. If he's seeing and attorney and wants a divorce, then what's the point of you visiting the gravesite at this time? You can go to visit on your own when you want to.
Sometimes, we have to step back and protect ourselves from further hurt and I think this is one of those times. Be polite when declining the offer...but it's time your h come to realize what life will be like w/o you being there for him.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
