Tolkien had Elvish “hair” words that began with fin- for most of his life, but the details of their derivation evolved. The earliest form of this root was unglossed ᴱ√FIÐI [FIÐI] in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. findl “lock of hair” and ᴱQ. firin “ray of sun” (QL/38). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. fiðra “collective hair” and {findel >>} G. finn “a lock of hair” (GL/35), the latter an element in the name G. Glorfindel “Golden Hair”, a name that retained this form and meaning for all of Tolkien’s life.
In The Etymologies of the 1930s Tolkien had unglossed ᴹ√SPIN with derivatives like ᴹQ. finde “tress, braid of hair” and N. finnel “(braided) hair” (Ety/SPIN). He made a point that this root was distinct from ᴹ√PHIN “nimbleness, skill” (Ety/PHIN), a distinction he reemphasized in later writings as well (PE17/17, 119). The additional form ᴹQ. fine “larch” (a species of tree with needle-like leaves) indicates the 1930s root was not limited exclusively to “hair” (Ety/SPIN). The root √SPIN appeared a number of times in Tolkien’s later writings, variously glossed “single hair, filament” >> “fine thread, filament” (PE17/17), “lock, tress of human/elvish hair” as extended √SPIN-ID (PE17/119), or “single hair” (PE17/185). However, in The Shibboleth of Feanor from 1968 Tolkien gave the root as √PHIN “hair” (PM/340).
Neo-Eldarin: For purposes of Neo-Eldarin, I would ignore this 1968 change of √SPIN >> √PHIN. Furthermore, I would assume √SPIN referred to a single hair or other kind of filament, and extended √SPINID referred to a tress of hair.
A root appearing in various notes written around 1957 connected to the mind (PE17/145, 155, 189; UT/400; VT43/16). Its most notable derivatives are Q. indo/S. ind “(state of) mind”. In The Etymologies of the 1930s, ᴹQ. indo “heart, mood” and N. ind “inner thought, meaning, heart” were instead derived from the root ᴹ√ID; this 1930s root was unglossed but had derivatives connected to both thoughts and desires, such as ᴹQ. íre “desire, longing” (Ety/ID).
Tolkien introduced a new derivation of indo from √IN(ID) in Quenya Notes written in 1957 (PE17/145, 189):
> IN-I-D, mind, inner thought. These refer to the movements or activities of the fëa or “spirit” (rational soul). indo, inner thought, in fea as exhibited in character or [?personality]. indóme, settled character, also used of the “will” of Eru (PE17/189).
The root √IN- “inmost heart, thought, mind” with extended forms √INID and √INIS also appeared in contemporaneous Notes on Names (NN), again as the basis for indo (PE17/155).
Tolkien coined yet another etymology of Q. indo in notes written in 1969, deriving it from √NID “force, press(ure), thrust” (PE22/165). This new derivation doesn’t necessarily invalidate √INID, but rather gives a new basis for the root itself as an vocalic augment of √NID rather than an extended form of √IN-.