So, "taflen" is leaflet, but is used as "sheet" in some contexts. Does this mean that "taflen" also translates as "sheet of paper"?
It usually refers to a leaflet or handout - a sheet of paper with something printed on it.
For examples, look up both taflen and 'sheet' on the reference site termau.cymru
Do you think that this might come from 'taflu', 'to throw'?
You might find an etymology and cognate words if you look it up in the on-line Geiriadur Prifysgol Cymru.
Good thinking. So no, when it means 'leaflet'. It's a borrow from Latin 'tabula', meaning 'tablet'.
But yes when it has it's other meaning 'catapult'.