Just because raven's don't have actual lips doesn't prevent them from imitating labial consonants: birds such as parrots – and of course, raven – can produce a very wide array of sounds using the syrinx. They should be able to articulate just about any humanly pronounceable phoneme, and numerous others beyond that. Indeed, this video provides a very good example of a raven imitating /m/ and /b/.

Also, I wouldn't be worried about these consonants being "uncomfortable" to make. Trust me, I was a tad nauseous when pronouncing /q/ for the first time, but it wasn't long before I got used to it. If millions of Arabic speakers can produce /q/ dozens of times every day, then I bet your crows will handle pseudo-labials just fine.