As the crebain were birds of Middle-Earth, I assume their name would've been adopted into Quenya as a loanword - but how would it have been adapted? *ravan? *karwan?
I see in Eldamo that the word has been extended with the meaning "raven", which makes sense in a way.
Q. exhilic [ts] became [ss]; [ts] > [ss]
In the Outline of Phonology [OP2] from the 1950s, Tolkien mentioned a late Exhilic Ñoldorin sound change of ts > ss:
but in Exhilic N. ts > ss (OP2: PE19/85).
This sound change was an addition in red ink (PE19/85 note #76), but it was also mentioned in the notes on the Feanorian Alphabet from the 1940s among “post classical” sound changes:
Q. initial [ŋ] became [n]; [ŋ-] > [n-]
In Tarquesta [TQ] pronunciation an initial velar nasal ñ [ŋ] was often pronounced as a dental nasal n, especially among the Vanyar. This sound change was not reflected in spelling. Tolkien mentioned this phonetic development in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:
Q. [rs] became [ss]; [rs] > [ss]
In Tarquesta [TQ] pronunciation there was a tendency to unvoice r before s and simplify the pair to ss. Supposedly this effected r of all origins, and also s < þ. Tolkien mentioned this phonetic development in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:
Ké lamyas *úsávima, mal *autamor akárier terkénima kauma ya pole fanta engwi kata sa (lan apantas i *katamen) ar vista te lakénime - laqui nante ambena i kauma, itas eke mon hententa te...
Maybe it sounds hard to believe, but inventors have made a transparent shield which is able to veil things behind it (while showing the background) and turn them invisible - unless they are closer to the shield, in which case one can spot them...