Ac including fan cuts off frequently. Reboots when system decides

no,just dirty air in your cabin.

Cabin Filter Replacement: Ford F-350 Super Duty 2008- 2016
My research indicates that your vehicle does not have a cabin air filter (also known as a pollen or AC filter). Vehicles without cabin air filters typically have a plastic mesh that prevents leaves and other foreign objects from entering the HVAC system. This mesh is part of the vehicle and does not need to be changed.

Turn on AC with engine running and after a few seconds all fans should start up if working!

It's normal for that to happen.It uses AC system to help remove condensation for defrost. You'll notice it doesn't happen on heat, only defrost. AC may be slightly low on freon causing it to cycle on & off more frequently.

It is possible that the pressure cut off switch is not working and so is making the compressor work continuously till the cooling pipes are frozen which eventually is reducing the cooling of the system.
hence have the cut off checked but as a trial run the car and manually cut off the ac when the cooling is good enough- use the highest blower fan speed within the cabin. If the compressor now works OK then this switch is not cutting off when there the pressure build up , the thermostat also is a suspect in this case.

charging AC has nothing to do with ur fan. if its a cabin blower u r talking about then open up the blower compartment and see if something is blocking the blower by functioning. if nothing is blocking the blower then it would be blower motor to be replaced.

Usually the cabin filter is located under the air draw that is directly below your windshield wipers. The air draw is the vents next to the windshield. Access is normally a few screws or bolts. Then replace the filter.

If the cabin temperature is remaining cool, the cycling of the compressor is pretty much normal as long as it isn't happening frequently and allowing ac inside to not cool. Operation is controlled by sensors that detect both cabin temp and system pressure.
If it is not cooling correctly one of those sensors may need to be replaced, or there is a partial restriction that is causing incorrect pressure readings..

Either AC compressor or AC clutch failure or both highly likely as Kia Sedona has notable history of this

Sure, one might trace the wire that connects to the compressor, and while its challenging to reach without removal of both fans, it does follow upward into the engine wiring harness. At that point its much more convenient to push a needle into the wire and connect a meter to it to see if it is energized when AC selector on, cabin fan on and cabin thermostat set to coldest setting.

If no voltage then check fuses and try swapping idential relays in the engine compartment box to be sure. If voltage present, then assume the worst.

More important is why the failure. I'm convinced a design error by Kia relative to the suction or low pressure hose is the root cause of such frequent AC compressor or AC clutch failures. You can buy a new Kia OEM suction hose, position it for to install and observe immediate kinking and or collapse. Kia has likely long been aware of this problem but never conducted a recall. You can solve it permanently, toss your existing suction hose and buy this one: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=181880844799

If you buy a "Tough One" brand replacement compressor and filter/dryer together from Advance Auto Parts it has a lifetime replacement warranty. No other compressor is sold with a lifetime warant for a Kia Sedona, not even from Kia itself.

I don't see a dirty cabin air filter causing your AC to fail - the cabin filter isn't connected to the AC system - it's in the vent feed from the outside, where air enters the car to be cooled. Having it filthy would only cut down a bit on the volume of air coming into the car, not on how well the AC system cools it. If the blower fan had a problem, I could see it possibly causing a failure, but I've never heard of a blower fan resistor - maybe a fan speed switch failing could cause it (that's known to happen with Audi cars), but Saab is somewhat different in their approach to car design, so I suppose it's possible. If nothing else, have them pull the service manual and show them the "blower fan resistor".

First off though, since you have your car back, I'd take it somewhere else, preferably an independent shop that specializes in European cars, and have them diagnose it. If they come up with something drastically different, I'd contact this place that you've been dealing with and let them know what has been found, and offer them a chance to make it right by you. Let them know that you may pursue legal action otherwise, as it's been apparent that they're blowing you off and don't really know what they're doing.

Did you get a return call yet? What did they say?