All good questions, Brian. But I think that the answers are closer to 100% than many people imagine. Though difficult to prove.
The marriage % is perhaps subject to the most "little knowledge is a dangerous thing" statements. People throw around statements like "People didn't get married until the woman was proved to be fertile" (i.e. she was pregnant). Rebecca Probert's book "Marriage Law for Genealogists" does some analysis and finds no meaningful evidence of this in the area and era she looked at.
The problem is that we are always moulded by the examples we find in our own families - hence if we get missing baptisms, we assume that it's widespread. For what it's worth, I've been intrigued by the high percentage of children who
are baptised. In one family, the children weren't - then, all of a sudden, nearly the whole family got baptised in one session - I suspect that was because their last living parent had died, the children went into the custody of other relatives who found out that they hadn't been baptised and remedied the omission swiftly. Plenty of cases where I can't find baptisms, of course - one family I'm looking at seems to be totally lacking baptisms - then I found that one marriage was Roman Catholic, so I suspect there's the answer in that case.
I reckon that 99.9% of people were buried in graveyards - where else?
What's just as likely to account for missing people, I suggest, are
- People who move - I think the population was much more mobile than we imagine;
- Records that are not kept or lost later - Bunbury PRs are severely deficient in the early 1700s according to some sources, e.g.;
- Errors in the originals - one of my ancestors in Acton (the one near Nantwich) was definitely recorded in error;
- "Lies" in the originals - my possibly Catholic family includes a will that is a masterpiece of misinformation;
Clearly genuine omissions are in there somewhere and we can speculate as long as we like (because it's good fun!) but my gut feeling is that they don't explain the
majority of issues. Probably omitted baptisms are a major issue just as significant as the ones above; omitted marriages maybe. I don't think omitted burials are significant.
Or that's my Sunday morning speculation!