While the word is written enedh in the Etymologies, it seems that Tolkien considered and reconsidered its form throughout his life. Late rough jottings (c. 1968), as well as the text of letter no. 168 and the fact that the toponym Enedwaith was never changed on the LotR map, seem to imply that ened is the (most) definitive form. See also the possibly related preposition ned , which has sometimes been suggested to mean "in" (while again a prefix nedh- is seen in the Etymologies)
Noldorin
nest
noun. ?heart, core
nesta
noun. ?heart, core
enedh
noun. core, centre, middle
hûn
noun. heart (physical)
nethra
noun. ?heart, core
A word that is an element in N. Forannest as the name of the north gates of Rammas in drafts of The Lord or the Rings (WR/354; 357 note #17). It had the forms nesta, nethra, nest and the difficult-to-read gloss “heart, core”. It is probably related the root ᴹ√NED “middle, centre” from The Etymologies (Ety/NÉD).