The usual Quenya word for “woman” or more exactly a “female person” of any kindred (PE23/87), in later writings appearing as both nís (MR/213, 226, 229) and nisse (VT47/18, 33). Even in the cases where its singular was nís, its plural form was given as nissi, indicating a stem form of niss-. In rough notes from 1968 Tolkien said “The monosyllabic nouns (especially those with only one stem-consonant) were a small dwindling class often replaced by strengthened forms (as nis- was [by] nisse)” (VT47/18).
Thus it seems the ancient form was ✱nis- from the root √NIS, which like its male counterpart Q. nér “man” inherited a long vowel from the ancient subjective form ✱nīs. But the voiceless s was felt to be intrinsic to word, and it was thus strengthened to niss- in inflected forms to avoid the sound changes associated with an isolated s. From this a longer form nisse was generalized. In practice I think either form can be used, with singular nís being preserved by analogy with nér. However, I think inflected forms are probably all based on nisse, such as genitive nisseo “of a woman” rather than ✱✱nisso.
Conceptual Development: In The Etymologies of the 1930s Tolkien had both ᴹQ. nis and nisse “woman” derived from the root ᴹ√NIS, with plural nissi in both cases (Ety/NIS, NDIS). He explained this variation as follows: “nis was a blend of old nīs (nisen) and the elab[orated] form ✱nis-sē” (EtyAC/NĪ¹). Hence it is was essentially the same as the scenario described above, but in the 1930s the long vowel in ancient nīs did not survive in the later short form nis. Quenya Personal Pronouns (QPP1) from the late 1940s had nís “woman” with a long í (PE23/85).
In Quenya prayers of the 1950s, Tolkien experimented with some alternate plural forms nínaron [genitive plural] >> nísi [ordinary plural] (VT43/26-29, 31), the former apparently representing a variant singular form ✱nína, but in later writings plural nissi was restored.
This root first appeared in The Etymologies of the 1930s as ᴹ√NIS “woman”, an extension of ᴹ√NĪ “female” (Ety/NIS). It also had a strengthened form ᴹ√NDIS, unglossed but apparently meaning “bride” based on its derivatives ᴹQ. indis/N. dîs of that meaning (Ety/NDIS). Unstrengthened ᴹ√NIS seems to have survived only in Quenya as the basis for ᴹQ. nis (niss-) “woman”, but this word was also blended with ✱ndis-sē to produce a longer form nisse of the same meaning.
In Tolkien’s later writings, both short Q. nís and longer nissë appeared as words for “woman” (MR/213; VT47/33) and Q. indis reappeared as well, though glossed “wife” (UT/8). As primitive forms, both unstrengthened √nis (VT47/33) and strengthened ✶ndī̆s “woman” also appeared in later writings, the latter given as the feminine equivalent of ✶[[p|n[d]ēr]] “man” (PE19/102).