At present, all I’m getting from this is that journalists need to be more precise with their language.
Both words mean “the greater part of”. They are, by definition, vague. The only proportion I think of when reading either is that more of that category than the other(s). In the case of 2 choices, somewhere between 51% and about 90%. I do tend to draw the conclusion that the writer couldn’t be arsed to find the specifc number or range.
If I had any context, I’d probably use that to help. for example, if I knew that the rioters were 95% Muslim, then I’d apply that percentage to my assumption of the kill distribution.
In this case, I’m answering the poll purely from a linguistics standpoint. When I read “mostly” or “mainly” I tend to assume about 65%. The words only require 51%, but I personally would describe anything between 40-60% as “about half.”
I would not use those terms for anything more than about 85%. If I wanted to communicate that, I’d say “the vast majority” or “almost all” instead.