Wikipedia has a basic article about how Welsh patronymic naming works http://en.wikipedia.org/wiki/Welsh_surnames
and more information from Welshleigh.org: http://www.welshleigh.org/genealogy/welshnames.html
WikiTree's style guide says to use the name/spelling the person in question would have used, so "Ferch" would be considered more proper than "Verch" because ferch is the original Welsh spelling. In Welsh, a single "F" is pronounced like a "V", so English speakers may have written it as "Verch." (To get an "eff" sound in Welsh, "FF" is used.)
As for women with "ap" in their names, it seems like there was a period of change and adjustment in Welsh surnames (names like ap Rhys becoming Price and so on), so the best thing to do for these is to double check primary sources or do more research into what the style for names was within her lifetime.