Bartrum does not list who was the mother for Cadwgan's children, although he does list Cadwgan's wives (discount Angharad ferch Llawr, as she married Cadwgan ap Elystan of Powys) : https://cadair.aber.ac.uk/dspace/bitstream/handle/2160/5047/elystan....
Darrell Wolcott says that he has no strong opinion on the matter at this time. So:
Dwnn i, 271 shows Hoedliw to be the son of Efa ferch Gwrgan. Dwnn i, 313 shows all of Cadwgan's children (12 sons, 4 daughters) to be the children of Efa ferch Gwrgan, including Llewelyn ap Cadwgan. Dwnn i, 332 says Idnerth is the son of Efa ferch Gwrgan, as does Dwnn ii, 152.
Dwnn i, 313 assigns no children to Jane ferch Brochwel, nor does Dwnn ii, 152. Only Dwnn i, 139 assigns one child, Llewelyn ap Cadwgan (see above paragraph for same ) and misnames her as Elen.
Here in Geni we have assigned no children to Efa ferch Gwrgan. It looks like all children have been assigned to Jane, except Constance Wen, who married Efa's half brother, Iestyn ap Gwrgan ( so highly doubtful that Efa was the mother of Constance Wen.)
So I'm throwing this out there to see if anyone else has any source material to shed further light on this.