Quoting Patrick H. Wynne:
> [If _Singollo_ is simply a contraction of _Sindicollo_, then we
> must wonder why, e.g., Fingon's Quenya name _Findekáno_
> (XII:345) did not similarly contract to **_Fingáno_? At the very
> least, it seems probable that if _Singollo_ is in fact a contraction
> of _Sindicollo_, it was not due to normal phonological processes
> in Quenya, but was a later, Exilic modification of the original
> Quenya form under the influence of S. _Thingol_. -- PHW]
I don't think anyone is saying that _Sindicollo_>_Singollo_ was a
*regular* development. It does not seem like Quenya compounds
normally underwent such contractions.
The possibility exists that the variant Quenya forms represent
various ideas about the exact derivation of Sindarin (Noldorin)
_Thingol_. In _Etym._, the name is referred to THIN and ÑGOL
(with the meaning "grey-wise" rather than "greymantle"), which
roots might every plausibly underlie a Qenya formation _Singollo_,
although the exact derivation suggested under THIN, from
Doriathrin _Thind_ (Q cognate _Sindo_) and _gol_ (Q cognate
_nóle_), does not suggest it.
[Nonetheless, Q. _Singollo_ is explicitly glossed 'Greymantle'
in the later _Quenta Silmarillion_: "Elwë Singollo, which signifies
Greymantle" (X:163). -- PHW]