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)
Sindarin
enedh
noun. centre, middle, centre, middle; [N.] core
ened
noun. core, centre, middle
ened
noun. centre
enedwaith
proper name. Middle-region, Middle-folk
Wilderness east of the river Gwathló mentioned in The Lord of the Rings appendices (LotR/1089) and appearing on its map. This name is translated “Central Wilderness” (PM/328) or “middle-people/region” (Let/224), a combination of ened(h) “middle” and the lenited form of gwaith “people, region” (SA/gwaith, Let/224).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name first appeared as N. Enedwaith “Middlemarch” (TI/296). Elsewhere, the Noldorin word for “middle” was given as N. enedh < ᴹ√ENED (Ety/ÉNED). Tolkien may have used a d instead of a dh because he did not want to confuse his readers with this unusual combination of letters. There are other examples of this in the Lord of the Rings drafts, such as N. Caradras for S. Caradhras, N. Caras Galadon for S. Caras Galadhon, N. Fanuidol for S. Fanuidhol, N. Galadrim for S. Galadhrim.
Some examples of d >> dh were corrected in published version of The Lord of the Rings (Fanuidhol, Caradhras), others not until the 2nd edition (Caras Galadhon, Galadhrim), but Enedwaith has never changed in the published text during Tolkien’s lifetime. Nevertheless, in private notes Tolkien sometimes wrote Enedhwaith (PM/328, VT42/7, NM/379) and in one place even stated that Enedhwaith was misspelt on the maps of The Lord of the Rings (VT42/20).
In later writings, Tolkien vacillated between ened and enedh as the proper Sindarin word for “middle” (for example, VT41/16 vs VT48/25). It appears he was unsure how to address the issue presented by this name, whether the name itself was incorrect or whether he should revise the Elvish words for “middle”.
enedh
middle
enedh (core, center), pl. enidh
enedh
center
1) enedh (core, middle), pl. enidh, 2) nest (heart, core), pl. nist
enedh
core
1) enedh (middle, center), pl. enidh, 2) nest (heart, center), pl. nist
enedh
middle
(core, center), pl. enidh
enedh
center
(core, middle), pl. enidh
enedh
core
(middle, center), pl. enidh
enedhin
midyear
enedhin (VT45:27)
enedhin
midyear
enedhin (VT45:27)
enedhren
adjective. central, essential
enedhin
midyear
(VT45:27)
nest
center
(heart, core), pl. nist
nest
core
(heart, center), pl. nist
@@@ Tolkien vacillated between enedh (< √ENED) and ened (perhaps < √ENET) in his later writings. In HSD/ened, it is suggested that ened is the likely “final” form based on Enedwaith as it appears in The Lord of the Rings and as discussed on Let/224. I prefer enedh for better compatibility with its Quenya cognate endë as in Q. enderi “middle-days” (LotR/1108, endë + ré “day”). If the Sindarin form were ened < √ENET, the corresponding Quenya form would instead be entë. Furthermore, in a general discussion of the uses of dh on VT42/20, Tolkien states that “Enedhwaith is misspelt”, possibly meaning that the normal spelling (Enedwaith) is not strictly correct.