Last year I was in exactly this position. I had grass hydroseeded in the spring and didn't want to kill it with my display. I wound up placing my tombstones right at the edge of the mulched flower beds. I had some standing skeletons and some kneeling ones in the grass and I moved them just a smidge after 2 weeks. The hard part for setting up is the overall 'who goes where' so although initial set up took longer (the yard changed a lot) it was pretty quick to move things 6-12" to the side.
We also set up the first weekend in October and take down first Sat in November. I think 4 weeks on baby grass is asking a lot of it. This year I am not so worried but last year ...
Good luck!