To add to Nicholas's excellent answers to mb0521's questions:
- according to witnesses (and Andrew Birkin's research), Michael and Rupert greeted friends on the way to the pool and seemed in a good mood, which doesn't sound like the behaviour of someone planning to commit suicide
- as Nicholas said, there was no foul play suspected, so no need for a full scale post mortem, which wouldn't have been standard procedure at the time.
- Michael's grave is in no worse state than some of the others in the churchyard, or other old cemeteries in London. It's sad, but unless descendants live nearby or can afford to maintain the graves, there's little that can be done. The only surviving descendants of the Llewelyn Davies boys are Nico's daughter Laura, who must be over 80, and her son, who live outside London.
- as far as I can see, there's nothing odd about the spacing or the wording on the stone. This seemed pretty standard for the time.