Zimmer's contribution was basically the opening credits theme, which was partially reused as the closing cutscene track. None of the other tracks in that OST used any cues from that track.
I'm 95% sure that you're wrong, but I can't cite any right now - I'll give the OST another listen and get back to you tomorrow.
I routinely turn off the music or play my own stuff instead of listening to the games' music because how it just sounds like auditory wallpaper. There's few tracks I'd say I'd want to listen to or that can conjure up any sort of emotional response when separated from the games.
I think it depends on the track. The mission music in ME2 (and maybe even ME1, except Ilos, Eden Prime, and Virmire) did tend to be pretty forgettable. To the point where they reused some in ME3, my reaction was "dammit, they're reusing that from ME2, but I forget where").
But the "cutscene moments" - reveal of the Citadel, the Spectre induction, the destruction of the real
Normandy, the travel through the relay, just to give a few examples - never fail to conjure an emotional response for me.
Well, different strokes for different folks, I guess.
Speaking of music, does anyone else simply leave the sound system OFF in your quarters for all of ME3? God, talk about some god-awful sounding crap from that thing. It was great in ME2 since you could choose from a number of ME1 tracks (I always set it to that epic music from when you first arrive at the Citadel) why not have it play ME2 tracks in ME3?
The ME1 tracks it reused were (IIRC) all composed by composers working on ME2. But with Jack Wall passing on ME3, and Sam Hulick a fairly late addition (at least as far as the public was concerned), the audio directors had probably already picked out stock tracks before they knew they'd have ME2 ones available for cheap/free reuse.