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: