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.

Quenya Phonetics P43: exhilic [ts] became [ss]

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:

Quenya Phonetics P41: initial [ps], [ks] became [ups], [iks]

Q. initial [ps], [ks] became [ups], [iks]; [ps-|ks-] > [ups-|iks-]

Late in Tarquesta [TQ] the initial combinations ps-, ks- came to be pronounced ups-, iks-, a sound change described in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s: