I am interested that the FAA approved the installation.
Whilst we might not like it the theory behind what gets hard wired into the panel or doesnt is safety. Are the electronics sufficiently robust and resistant to short circuits and fire, do they represent a physical hazard in the cockpit or impinge on the pilot operating the controls or his view, and does the equipment do what is intended. I can see how you might mount the more basic version of the Xaon in the panel (and indeed Zaon even sell a kit) but I have no idea how you would panel mount the XRX given its shape.
Zaon offer some guidance in their installation manual and discuss whether the FAA consider the installation a major or minor mod. Clearly you might persuade an A&P that the Zaon is a minor mod or the FAA that it is a major mod. I very much doubt you would persuade our dear CAA on either count. Having persuaded either the issue then arises what aerial(s) is / are used, where it is / they are placed and what effect other avionics and panel screening will have on the unit if external aerial are not used. You will need even more luck with the FAA to gain approval to have one (never mind the ideal two) external aerials bolted on the airframe. All in all I can think of all sorts of problems installing a Zaon in a certified aircraft and insurmountable problems if the aircraft were pressurised. Moreover I doubt even Zaon would claim this was the purpose for which the unit was disgned - you have to read a fair amount of tongue in cheek when they talk about how you might gain approval.
However, assuming for a moment you overcome all these problems, as pilot in command you then must consider what you do when an approved piece of equipment is not functioning. If you get 1 in 100 ghost alarms is it faulty? What about 1 in 50, or as you suggest 5 in 10 or more? At some point you placard the equipment as U/S and the shop either fixes it, the equipment is removed, or permanently maked u/s. You do that not only for yourself but for the poor sod who is next to fly the aircraft and entitled to think that everything not placarded or noted in the tec log is working,
For all of these reasons I think that anyone who panel mounts one of these units in anything other than a home built is attemting something that was never really intended and without at least one surface mounted external aerial is asking for all sorts of problems.
I reckon both units are designed to "sit" on the glare shield, where the aerial has the best chance of doing its job, wired into an accessory socket if you wish and correctly calibrated to the aircraft as recommened by Zaon.
In short if you gained a field mod including approval for at least one external surface mounted aerial I suspect your A&P had little idea what he was doing and if there were no external aerials included in the installation then he should be shot if only because you will be left with a rather redundant and ugly hole in the panel which would only be useful as a cig. packet holder - but fortunately they have been banned in the cockpit as well.
Sorry but the more I hear the less I am surprised that you encountered as many problems as you did which sort of comes back to my original point that if the issues arent understood dont be surprised if you dont like the results. One other lesson I have learnt is engineers get it wrong like the rest of us, just because they have a bit of paper doesnt mean they know what they are doing, so treat anything that doesnt appear to be working with a healthy degree of sceptism and dont assume that because it was "professionally" installed it is the equipment that is faulty and not the installation.
That's all you really need to know.
So you are at least right with one thing that is all I needed to know to make a pretty fair guess why you had so many problems.
That solved the thread has being going too long so I will leave it at that.