This root was first mentioned in notes on the Adûnaic language from the 1940s with the meaning “heavens, sky” (SD/414), and √MENEL appeared in Common Eldarin: Noun Structure from the early 1950s with the gloss “heavens, firmament” (PE21/84). Both Q. and S. menel appear quite regularly in Tolkien’s writings from the 1940s forward.
In the 1940s conception, √MENEL was ancient enough that it was able to influence the Adûnaic language indirectly via Avari, producing Ad. minal or minil (SD/414). However, in the 1950s Tolkien said “thus it is reported that √MENEL ‘heavens, firmament’ was devised by Quennar of Túna in ancient days while both Noldor and Vanyar dwelt in Tirion, and so is part of the Vanyarin (Quenya) speech and of Noldorin, but not of Telerin” (PE21/84). In notes from The Road Goes Ever On published in 1967 Tolkien said of menel: “The word was a Q. invention from men (direction, region) + el (the basis of many star words)” (RGEO/64). Thus it seems his later idea was that it is was originally a Quenya-only word, later borrowed into Sindarin.
Note, though, that Ad. minal remained in later writings as well, as an element in the city name Arminalêth (PM/145). These two competing conceptions can be reconciled if Ad. minil is likewise a loan word from Quenya in the First Age, adapted in a period where ancient Adûnaic did not have short e, and then later altered to minal. This was, in fact, an alternate scenario that Tolkien considered in the 1940s, though in that period he considered it more likely that √Ad. MINIL was more ancient than that (SD/414).