I then entered some descent information on the FMC
Don't be surprised then when the FMC is given 'better' descent info (w/v) that it may pitch down to regain the Vertical profile! Why were you programming the descent at that point?
At the end of the day, if you see a situation develop( like an overspeed), your first reaction should be to fly the aircraft (via a/p modes - select v/s or flch, or handfly) but not to reprogramme the FMC!
I'm sorry, but you have to remember who is in control, you or the computer. I have little sympathy for you (and even less for the captain).
Having said that, your operator sounds like a nightmare, but I'm sure you'll learn your lesson!