I'd take esreverlluf's answer.
1. If you're in VNAVSPEED it will not make speed/altitude contraints unless it
RECAPTURES VNAVPATH. Your question stated "in VNAVSPEED" so the answer is no.
3. Yes, as long as it stays in VNAVPATH. Change the speed in the FMC, understand why, and what, changes occurred to the descent profile, and if it transitions to VNAVSPEED you should be able to figure out how to regain VNAVPATH(piloting 101)
Depending upon what speeds you enter into the FMC descent page it might drop out of VNAVPATH. Selecting slower speeds, while near TOD or in the descent while on profile(VTI=zero), will change the profile descent. If it drops out of VNAVPATH(not uncommon with speed reduction) your speed/altitude constraint protection is gone.
If you use speed intervention on the MCP it will drop out of VNAVPATH.
This is like public math....hopefully I'm correct.
