Eleanor de Mortimer m. Higyn ap Llywelyn of Ceri/Kerry

I have found several Welsh sources stating that Eleanor de Mortimer "daughter of Sir Roger Mortimer of Wigmore" married Higyn ap Llywelyn ap Griffith ap Higyn, steward of Montgomery and Kerry (Ceri). I am pretty sure that Eleanor would have been illegitimate, and am trying to figure out which of the many Rogers de Mortimer could have been her father.

Higyn's daughter, alternately named as Elena or Jane, married Meredith ap Adda Moel, who served as steward of the same lordships his father-in-law had held and was named in a charter by Edmund de Mortimer, 5th Earl of March, in 1416 (shown in the Montgomery Collections, vol. X, p. 59 - 60). Unfortunately, I can't find any other dates to pin to these individuals.

The charter was signed in 1416, and Meredith had served faithfully for some time before that. Assuming he and his wife were about 36, that would give them birth dates around 1380, and if his wife's mother was in her 20s at the time of her birth, then she would have been born sometime in the 1350s. In this case, she would be a possible daughter of Roger de Mortimer, 2nd Earl of March (1328 - 1360), whose known children were born in the 1350s.

I can't find a reasonable way to make the math work for any of the other Rogers who were lords of Wigmore, but I am open to suggestions from any Mortimer experts out there.