Update (like you care but just venting/reflecting):
We are a week into it. I had a little talk with him during our a drive for me to drop him off in upstate NY to visit his brothers and sisters. He's been put in his place a bit and my wife and sister-in-law are amazed on how his manner has changed since our father-in-law to son-in-law talk during the drive. (2 hr drive alone in the same car).
He's still a handful though, almost like taking care of another child. (He has some health & mental issues so it's hard to fault him for it. My best analogy would be Rain Man light)
All and all, it's been a pretty good visit so far. Grandpa gets to hang out with the kids and the kids really adore him - he's great with the them and that's priceless on it's own.
In a couple of weeks, he'll be flying back to Thailand (he left the States to retire there) and who knows if we'll see him again alive.
Tough it out, life is short. (way too short).