i don't get how active steering unit would wake up the car. i can see that the car is not going into full sleep mode...which if you do a search on bimmerfest you will find many with that issue especially if you have comfort access. not sure if you have comfort access. keep key far away if you do.
but steering unit causing it i find hard to believe. it's more like the computer is not going into 'deep sleep' mode. often time you can just check by looking at the power window switch to disable the rear windows. that green light should turn off. if it doesn't the computer is still active. i know on my car sometimes i hear it wake back up just by being in the garage....it's like it sensed me or something. eventually it goes back to deep sleep mode.
i don't know what the solution or cause of it is...i just recall reading these symptoms. i would get a second opinion...these dealer guys are doing bare min. to get to root cause. unless they put it in writing that they will reimburse on replaced part if problem persists - walk away. no dealer will admit to that. and how does battery and alternator run you 2300!!! WOW! second reason to walk away.
the more stuff they touch/replace other issues will crop up as things never go back together like factory. and the e60 is one complex car! along with the 7series.