After debuting as Spectrum (or something else) Monica can take over the CM name as a tribute to her fallen friend.
Yes, but NOT debuting in Captain Marvel 2. It's easy for superhero movies to become overstuffed trash cans by trying to stuff in supporting heroes from decades of comics along with villains to fight.
CM2 with Carol, Ms. Marvel, Spectrum and more than one villain would be as bad as Iron Man 2 at best - worse if Captain Marvel is itself an inferior film to Iron Man. It would also shorten Monica's MCU lifespan, because she'd have already made 1-2 contract appearances before CM3 rather than letting the universe and its contracts breathe.