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
lond daer enedh
place name. Great Middle Haven
enedh
middle
enedh (core, center), pl. enidh
enedh
center
- enedh (core, middle), pl. enidh, 2) nest (heart, core), pl. nist
enedh
core
- 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
nest
center
(heart, core), pl. nist
nest
core
(heart, center), pl. nist
The usual Sindarin/Noldorin word for “middle” is enedh derived from the root √ENED (UT/264; VT48/25; Ety/ÉNED). This is complicated by the fact that Tolkien wrote Enedwaith “Middle-region” on The Lord of the Rings map (LotR/1089; Let/224). It is my opinion that this use of d for dh originally reflected Tolkien’s normal representation of this letter Lord of the Rings drafts of the 1940s, for example 1940s N. Caradras and N. Fanuidol vs. later S. Caradhras and S. Fanuidhol.
Unlike those other representations of dh, Tolkien never corrected Enedwaith in the published texts, and in notes from the late 1960s he considered revising the “middle” to ened derived from √HENET (VT41/16). In other notes from this period he said Enedhwaith was misspelt ened (VT42/20), so I think ened < √HENET was likely a transient idea.
In compounds enedh is generally used as an adjective, such a lebenedh “middle finger” and Lond Daer Enedh “Great Middle Haven”. In The Etymologies this word was glossed “middle, centre” (Ety/ÉNED) and “core, centre” (Ety/NÉD), which makes me think it was a noun.
Neo-Sindarin: For purposes of Neo-Sindarin, I would treat enedh as a noun when used independently, and use [ᴺS.] enaidh as the adjective for “central, middle”.