If the system will continue to use consumer equipment, add the compressor/limiter as a separate, locked box -- beyond the control of an amateur DJ.

Also, explain how fast sound travels and that, even if all of the speaker outputs are time aligned, there will be acoustic delays between areas that might be audible. In some cases you may be able to cope with acoustic delays by adding audio delay -- similar to what is done in large live venues.

In some respects it seems like the customer wants to be in charge of the project and use you as the wire man. Unfortunately, the customer is attached to his inappropriate consumer gear.

I think that he will be best served by whatever matrix and control system that you like. Hopefully, the system will provide access to online music. I'm not using it and I don't see much mention of MusicCast on RC, but it seems like a capable system and it will have name recognition for the customer.

The 5.1 rooms will be a thorn in any case because the A/V units will add their own brand of processing delay. Sometimes this can be eliminated with a "Direct" setting that can be used during DJ events. Setting Direct Mode would be just another task for the control system or human who sets up for parties.