You have asked the $64,000 question. You have the two Helens wrong. The Helen
that married  John le Scot and m Robert de Quincey was the daughter of
Joan. The other Helen is not a proven daughter of Llywelyn at all.
You have assigned Gwladys to Joan instead of Tangwystl -- and I think that is
fair. It is 50/50 at best that you are right -- and with Joan's lands going
to Gwladys family and Gwladys dying at Windsor -- and the Mortimer genealogy
going through Joan to William the Conqueror in a 14th century manuscript
tradition, I would say the weight of the slim facts favor Joan over the Welsh
mistress. Since you have to enter a mother, I would keep it that way. I will
change my personal database from Tangwystl to Joan on the basis of this
Llywelyn ap Iowerth, Prince of North Wales, and his lawful wife, Joan, had several legitimate children, among them being Ellen, wife successively of John of Scotland, Earl of Chester and Huntingdon, and Robert de Quincy, Knt. Ellen's place as a legitimate daughter of Joan is proven by an entry in Calendar of Close Rolls, 1234-1237 (1908), pp. 538-539, where Countess Ellen is styled "our niece" [nepte nostra] by Henry III, King of England.
Recently, however, I encountered further evidence which supports blood kinship between Countess Ellen and the English royal family. Ellen had two surviving daughters by her marriage to Robert de Quincy, one being Hawise de Quincy, wife of Baldwin Wake. The record below states specifically that Hawise de Quincy was of the "consanguinity" (bloodline) of King Edward I:
"Date: 18 Feb. 1282
Order [by King Edward I] to the sheriff of Lincoln to levy £100 from the goods late of Baldwin Wak, deceased, by view of Aubrey de Wintelbiry, and deliver the same to Hawise, late his wife, for the expenses of his burial, and to levy also £40 therefrom by view of the same Aubrey and deliver them to her by reason of her consanguinity."
[Calendar of Fine Rolls, 1 (1911): 159].
Hawise de Quincy, wife of Baldwin Wake, and King Edward I were first cousins first removed, they both being descendants of King John.
Best always, Douglas Richardson, Salt Lake City, Utah