Sindacollo is according to Christopher Tolkien one of the names
of Elu-thingol in Quenya in his "Silmarillion" (1977).
But in HOME the name is always spelled _Sindicollo_; see HOME XII
The "normal" form of the Q. adjective according to "Quendi and
Eldar" (XI:384) is _sinde/thinde_, not _sinda_, which is a
personalised form of that adj. (cf. "Sindar : the name (
derived from (
) _sinde_" ibidem), being the name given to certain
Elves, even through _sinda_ is used as an adj. in the Namárie
elegy, cf. "sinda-noriello".
So _Sindacollo_ is not grammatically wrong in Q. as far as we can
With the name Q. _Melyanna_, this _Sindacollo_ is the last piece of
information in the Appendix of "The Silmarillion" (1977) that we cannot
pinpoint to a precise source in one of JRR Tolkien's linguistic writings.
Any chance of seeing this piece untangled in a future Vinyar
[I will keep an eye out for an attestation of either of these forms in
the mss., and will note it here if I succeed in finding them. -- PHW]