1.) As to distribution amps, in theory they provide some boost when spliting a low voltage signal several ways. I have a somewhat similar setup to yours with multiple megachangers and use a radio shack distribuion amps to split the analog channels directly out of the megachangers to feed the signal off to seperate zones feeding the digital signals to the 990 only. This has worked well for me, but I'm not technically adept enough to tell you if it's really necessary.
If we're speaking of the same device when we say distribution amp, I don't think the resistance of your speakers will impact it but will impact tthe power amp thats driving the speakers.

2.) I have an abundance of old receivers about so I use a Denon 3800 in my familyroom recieving signals from the megachangers in the home theater, the 990 itself, and local digital inputs from an Oppo DVD player and a Cabel box. This supports a 5 channel system in the family room and a 2speaker and sub in the Dining room. I also use an old Techniques reciever for a 3rd zone in the excercise room/library and patio. I find that having the ability to incorperate local sources in the remote zones is benificial in my situalion and the ability to control volume and manage bass locally is necessary.
3.) I don't have any experience with the Behringer. Receivers do the job for me but they're only practical because I happened to have them. They probably wouldn't be a practical alternative if they had to be pruchased.
4.)Gonk will probably clarify the "dimmer switch" issue, but there are devices generally called speaker selectors that take one set of speaker level inputs and feed multiple sets of speakers working some magic to keep the resistence up so that the amp doesn't overload. many of these have individual volume contrlos but do not amplify the volume so the controls can reduce the volume of the signal received, but not increase it. Such devices are used after the amplifier and before the speakers.
5.I wouldn't think an ICBM would be necessary in your zone 2. As I've said, I use receivers to provide bass management, though the crossover in a subwoofer should work just fine. The quality of the crossover will likely vary with the quality of the subwoofer. I'm not familiar with outside "rock" speakers or subwoofers but I would think the selection would be limited. You'll want a powered rather than passive sub and you'll want to look for one with a variable low pass filter if you purchase these seperately. If they come in a set with the speakers, they are probably already set with an appropriate crossover to compliment the speakers.