Firstly, a correction to a careless error I made in my original post: For Mary Barrie, please read (of course) Margaret Barrie.
Sorry, both, but the accidents were not too similar, were they? David, we have been led to believe, had been standing on a frozen lake when he was knocked over by a skater, whereas Natasha fell over during a skiing lesson, although I think we have not been told how she came to fall. As for the injuries sustained, and the subsequent deaths, of these most unfortuante individuals, whereas the circumstances of Natasha's situation are fully understood, those of David would seem to have been different and will probably remain unproven.
Without going into detail here, I think we can no longer be certain exactly how David came to bang is head on the ice, or even if he banged his head on the ice at all! In my opinion, the information on his death certificate, which seems not to have been gleaned, used or speculated upon publicly until 2005, gives no clue as to how David's brain became inflamed in the first place. The doctor made no mention of it having occurred as a result of a blow to the head, for example. While a head injury is one possibility, so too is infection, and so too is an autoimmune disease, as I explained in my ANON posts in July 2008.
From reading about what happened to Natasha, who died of an "epidural hematoma due to blunt impact to the head", it would seem that unless the pressure build-up, which occurs inside the skull and against the brain, can be relieved quickly, death usually occurs from within a few hours to within a few days, and almost certainly within 72 hours. (ie. not within several days; not within a week).