I would venture that your issues are all caused by your lack of knowledge or skill, and I mean that constructive, not to be mean. Drop by a bike shop and give the guy $20 to come assist you. you should be able to mount bikes so they touch yet do not cause harm. Pipe insulation over frame tubes helps a lot and is used even by those owning $10,000 bikes! Bungee things together so there is no clanking. Keep pedals out of the other bike's spokes. I have never heard of breaking a cable, something went terribly wrong if a cable gets broken (I assume you means the black calbe HOUSING, not the thin wire cable?)
Do you have a receiver hitch on the back of the trailer? I am not sure what you mean by "luggage rack". I assume a receiver hitch with some sort of tray inserted.
At the bike shop again, look at their "tray" type mounts. The ones that hold the bicycle wheels in cradles and hold again at the top of the tire.
The best is the One-Up bike rack, or "1UP". Each bike tray steps up one higher, so when you go up that steep driveway the rear of the rack does not drag as it will if all the trays are the same height. The One-Up can take 6 bikes I think. Each bike tray is modular and can be added or removed.