That link doesn’t seem to be related to fill factor. I was looking for something like this
http://dev.mysql.com/doc/refman/5.7/en/index-page-merge-threshold.html in Maria, but cannot find it. That link says that the page merge threshold used to be hardcoded at 50% free, hence my worst case estimate. It’s plausible that this is way too pessimistic in practice, and that sorting before loading results in relatively full pages. I just don’t know for certain.
I don’t think so - that seems to be about storing long column values in overflow pages, and we don’t have any of those here.