And a final disappointment...
I can no longer support my idea that Eliwlad is a sort of substitite term for the Ailithir epithet of Madog (Matoc) son of Sawyl Benisel of the North.
First - and most critically - the Welsh from quite early on had their own word tir, 'land, earth', etc., corresponding perfectly with the Irish word found as -thir in Ailithir. Thus we cannot justify the use of -(g)wlad in place of -thir. This is the one stumbling block all scholars can agree on. There simply is no way around it. Unless, of course, the poet has knowledge of Madog Ailithir son of Sawyl and chose to appropriate him because Uther happened to have a son named Madog. Then, to create a new character, he substituted gwlad for tir.
Second, we cannot ignore that fact that Eliwlad is presented as an eagle. This is an important point.
What we have in the 'Dialogue of Arthur and the Eagle' are
Uther Pendragon (dragon)
Arthur (arth, bear)
This listing of animals and birds is hardly coincidental.
Furthermore, the 'eagle of Eli' of the Cynddylan poem is too perfect a match for the eagle Eliwlad. As I've discussed before, Eliwlad is placed in the valley of the wood of Cornwall because in the Bodmin valley there is a Cutmadoc place-name. BUT, Powys, the kingdom in which the Eagle of Eli is found, was once known as Cornovia, a tribal name who root is exactly the same as that of Cornwall (Kernyw). I would see Eliwlad as a manufactured name 'Eli-prince'. While gwlad is Welsh is mostly used for 'land, kingdom' and the like, there are some examples of it being used along the lines of Irish flaith, 'prince.'
Less satisfactory from a strictly philological standpoint would be to identify the eagle in the oak in Cornwall with Mabon, one of the three predatory birds (wythaint) of Elei in the PA GUR poem. Mabon was identified with Lleu in Welsh tradition; the former's grave was situated in Nantlle, which is where Lleu took the form of the death-eagle in an oak. I have noted the presence of Mabyn (= Mabon) place-names near Bodmin: Tremabyn a few kilometers west of Cutmadoc and St. Mabyn further to the NW. It is not impossible, therefore, that Eliwlad is a corruption of Eleiwlad.
Much as I wanted to be able to link Eliwlad son of Madog with Madog Ailithir son of Sawyl, I now feel the former is merely another fictional character added to Arthur's family.