Don't think about using ANY consumer preamps or amps. They are not right for what's needed. If you use, say, a DirecTV receiver for the audio of a TV channel, the two channels will need to be mixed to mono.

Yes, the system will be mono. It will be 70 volt. 70 volt is not at all difficult to work with. It's just a different way to look at output power, speaker impedance, and balancing of volume.

It might be good for you to ask specific questions here in addition to looking for "more advanced" material.

You'll need to look into a licensed music source. The venue could get fined for using music that's not licensed.

Think seriously with the client about where the zone volume controls will go. They could just adjust the loudness of the sound, or, as some JBL product does, the volume control could be a device that adjusts volume in each zone as well as selecting the input for each zone.

DO NOT think that it will sound bad because it's 70 volt. Good 70 volt speakers sound good. We have the concept that 70V is crap because we were introduced to 70V systems via supermarkets, or gas stations with outdoor horns, where as much as ten bucks might have been spent on each speaker. Ten bucks doesn't buy quality in any kind of speaker. Check with Fred's favorite supplier about that. They can help.