Someone more knowledgeable than me will probably come along and shoot down this theory in flames but it's possible that the grave move glitched because Michael is one of the glitched characters in Pleasantview. The hood comes with 2 versions of him, one which is a stump file and another which is a full character files. Unfortunately the stump file Michael got linked to the urn and not the full character file one, possibly causing havoc when you move the grave. For those using cleaned up neighborhood templates this is not a problem because that template has been fixed and the proper character file is linked to the urn. But if you're using the original Maxis Pleasantview template, the "wrong" Michael is linked to the urn.
The other character that suffers from that glitch in Pleasantview is Skip Broke. The stump file Skip is linked to the Broke family, which is why Brandi's third child will always be a male clone of her. The game has no genetic information to draw on so it generates an opposite gender clone. Again, this is fixed in the cleaned up Pleasantview template, making the full character file Skip the genetic father of the unborn third child and making it possible for Brandi to have a girl.
But this is just my theory as to why Michael's grave glitched your lot. I know that he is glitched while Darleen is not so that was the first possibility that came to mind. I know you've been playing this hood a little bit. Maybe make a backup of it, get the cleaned up Pleasantview template (cleaned up NOT empty), make a new Pleasantview and start out by moving Michael's grave. If it borks, you just delete the hood and put back your backup, no harm done and no gameplay lost. But if it works and doesn't bork at least you know the cause. I know it's hard to start again but if this works then you'd probably be better off playing with the unglitched hood than continuing with a glitched one. You can get the cleaned up templates
here. The cleaned up Pleasantview is a fair way down in the first post so make sure you grab that and not the empty one, which is at the top. There are instructions on how to install them, too. Or you could just plonk N001 into your Neighborhoods folder. Make sure you first do a backup of the hood you've been playing.