Center channel level - if you are sure that your speaker system is properly calibrated, stay with it. You are providing the reference, not random consumer systems (unless the other systems you used to check your mix were properly calibrated and yours isn't). The other systems might have insufficient center channel speakers compared to your matched speaker, but again you are the reference here.
Normalization - I'm not sure if I'd use that term, but thankfully surround mixes generally (I can name exceptions) aren't squashed like most 2.0 CD mixes seem to be these days. IMO Make sure it's as loud as you can get without overs, and add a little 5.0 compression as it suits the mix, not for loudness. Loudness is provided by the consumer's volume control (as it should be for CDs, but that's another thread...).
5.0 mixing style - 2.0 and mono are one-dimensional - width of some sort, with no real height or depth (depth in a 2.0 mix is subjective IMO). The benefit of 5.0 stereo is that you add depth extending front and back, right and left. If you listen to a wide variety of 5.0 mixes, you will find a variety of approches to vocals. The extreme approaches include vocals dry in the center channel only, and nothing at all in the center channel. 5.0 mixes that work in the broadest ranges of systems, for me, bring vocals a little closer to the listener by having the vocal about 60% in the center and 20/20% R&L, taking advantage of the dimensional space. Also, build some reverb/effects around the vocal, not just putting reverb/effects in the surround channels.
One guitar - I've experimented with placing it equally in the Center Channel, R/C phantom, and L/C phantom, giving it size and dimension, but not the same as the vocal, but there are a lot of approaches.
Room mics - again, probably not strictly in the surround speakers, but centered somewhere between L/Ls and R/Rs (or even L/Ls/R and R/Rs/L to bring them a little toward the middle of the soundfield).
I believe it would be Fair Use to rip a few DVD Dolby Digital tracks to Nuendo for analysis and study before you proceed.