The sound change [nl] > [ll] was noted by David Salo in his book, Gateway to Sindarin (GS/§4.158), on the basis of the names Mithrellas (UT/248, PM/222) and Finellach (PM/351). Neither name is glossed, but the first seems to be a combination of mithren and lass. The etymology of the second name is made clearer by earlier (rejected) forms Finlachen and Finhenlach, the latter strongly indicating that the middle element is hen “eye” followed by lach “flame”. Additional support for Salo’s theory appeared after the publication of Gateway to Sindarin, in the (rejected) word caralluin, which appears to be a combination of caran “red” and luin “blue” (VT48/30).
Counterexamples to this phonetic development also appear, however. Salo himself noted that minlamad (WJ/311, UT/146) preserved nl. Both of the rejected froms of Finellach do not show this sound change, and there is the hypothetical “true Sindarin” form Gonlin (< ✶Gondō̆-lindē) for the hybridized Quenya-Sindarin city name Gondolin which likewise preserves nl (PE17/133).
If we assume these variations do not represent conceptual vacillations on Tolkien’s part, it seems likely that this sound change occurred after short vowels vanished at morpheme boundaries [mithren(ă)lass > mithrellass] but before middle consonants vanished in clusters [gond(ŏ)lind > gondlin(d) > gonlin]. In any case, nl > ll is probably no longer an active sound change in Sindarin, and minlamad is probably a late (or reformed) compound.
Conceptual Development: It is conceivable that the name Mindolluin might have originally been conceived of as N. mindon + luin “Blue Tower” (later glossed “Towering Blue Head”). If so, it might represent an example of this sound change from the 1940s.