Chriss - It would be my guess that using a pre-filter air injector would be a bad idea. I have the feeling that getting air bubbles on to the filter membrane with not be a good idea. Maybe someone with more knowledge will pipe in. That recommendation made by the service I must interpret as BS from a guy who has no idea what your device is or how it works. As you have found, you should not stop the cleaning cycle. The purpose of the cleaning is to keep the membrane clear. If you really really want to do something longer than the included cycles, you could probably use your own timer instead of the built-in one and set you own schedule. But I really would not recommend it. Some very smart engineers said that was how it should work. If you have a water supply problem not wasting rinse water, perhaps pipe it somewhere useful rather than just a drain.
It could be just a clogged or defective valve. It don't think the drain would be clogged while processing remains unaffected. You first need to decide if Dirty Harries view (as I remember it) is correct "Do you want to try it punk; do you punk". The Zenon can be drained and dissembled.
If you isolate the Zenon from both sides of the system (surely you have cutoff valves in the circuits), you can drain the water and possibly check the pressure compartment (I recall there is one in there) used for backflush. Then remove parts. Clean and test the discharge hardware. If the valve or solenoid have failed you will need to find some matching parts. I would do any repairs from the outside if at all possible without threatening the membranes. You can unscrew the top of the filter and access the membrane and whatever you have in the top layer (carbon filter or metal mesh). I would try not to annoy the membrane. If it becomes necessary to remove it to get to something else, be very careful.
A couple of things.
If your Zenon is intact, you don't need UV after it. Having the UV won't hurt anything; and it is a belt and suspenders solution.
It would be useful for you to describe your water, and the source. And what the various other devices are doing. I am not sure you have an optimal configuration. Describe the capabilities of each of the devices. Describe quality of water and the contents you are trying to remove.The other devices you have before the Zenon, probably don't play a part. But, I might make sure the water softener is operating properly and not leaking materials into the water. Double check you prefilter is not passing particles. While you are at it, and if you have some gauges on the system, you might outline the pressure drops from the well and various devices until the outlet of the Zenon. What flow rate do you need from the Zenon to you water system. Are you staying within the limits of the household flow demand. Most of these suggestions are probably not critical information, but would give the people here more to chew on. What I have said here is just the bunch of things that are probably not the problem, but information could be useful to you when troubleshooting or for helpers on this thread.
As an aside, I put a pressure tank on each side of the Zenon, with a backflow preventor between the Zenon and the house water system. It may not be an issue for you, but it allows you to have higher flow in the house than the Zenon should be asked to supply. The pressure the tank size will determine what you can have for flow in excess of the Zenon flow rate in the house. It works quite nicely.
Greg - I would like to know as well.