Basically, my biggest concern in multi-room, not necessarily multi-zone, so two zones should be sufficient. I also want the ability for both zones to play the same input, this should be possible, right?
Yep, same thing at the same time will work fine (and you will be able to adjust volume between Zone 1 and Zone 2).
Hook up my music server via the "Optical In" port, along with a CD player in another port.
I think you'll also have to hook them up with an analog connection if you want them to play in Zone 2.
Hook up the two pairs of In wall Speakers to the "A and B" ports on the Denon. (They will still be able to play the same source at the same time, correct?)
Yeah, the A and B will play the same thing at the same time at the same volume. However, there may or may not be A and B on that receiver. If not you can hook them up to the "Surround" channels and set the receiver's mode to "party mode" or "multi-channel stereo". In that mode, it will play pretty much the same material on the "Surround" channels as the "Main" channels. Check out the owner's manual on line for the details of those modes (I'm assuming they have it; I have it on my old Denon as well as newer preamps -- it seems like a somewhat basic mode these days). Connecting to the "Surround" channels will also take care of any concerns with impedances causing trouble at the receiver.
If that doesn't work, and there are no B jacks on the receiver, you can always hook them up in parallel (basically, just hook both pairs of speakers to the same terminals). This
can cause problems if the impedance of the speakers is too low to begin with. I'd look for speakers that are 8 ohms, as they will present a 4 ohm load when connected in parallel. If you are not playing at loud volumes, you will almost certainly be OK with a total 4 ohm load. If you start to strain the amp, there
could be problems. All that said, I've run lots of receivers with 4 ohm loads and never had a problem until they start to run out of steam.
Hook up the third pair in the other room to the "Zone 2". (Will I still be able to play the same music that is being played in Zone 1 through Zone 2 if I want to?)
Yep, will be able to select any input to be directed to Zone 1. You will be able to select any
analog input to be directed to Zone 2. If you want to listen to the Roku in both rooms, you will need to connect it with a minimum of its analog connections (for the Zone 2 connection). If you want, you can also connect it with an digital cable, but that will be only available to Zone 1.
Possibly add volume controls in each room.
Nothing wrong with that.
If I ever choose to add a fourth pair, would you guys recommend that I use a speaker selector and split the Zone 2 signal? I realize that i will only be able to play Zone 2 through those speakers.
That would be one way to do it. Again, I'm usually OK with running speakers in parallel, so I would at least
try to just hook another pair to the Zone 2 speaker level output. Also, and I'm not sure about this (check the manual), there may be a way to output a Zone 2 line-level signal to another power source (your other, old receiver, for example). You would then hook that fourth pair of speakers to that receiver.
Also, you said I shouldn't have to worry about the impedance load?
As described above. How loud do you think you'll be using this setup? If you're going to be pushing it, I'd have more reservations.