Furthermore, "determination" of interference actually means that if the manufacturer has not certified the equipment to DO160 standards, it is up to the operator to do so, before it can be used. Switching it on and listening for crackling over the comms is not sufficient (nor even likely to find) interference of the kind tested for.
I think you're extrapolating a bit there. This is the actual relevant paragraph from the FAA link you quoted above:
(5) Any other portable electronic device that the operator of the aircraft has determined will not cause interference with the navigation or communication system of the aircraft on which it is to be used.
(c) In the case of an aircraft operated by a holder of an air carrier operating certificate or an operating certificate, the determination required by paragraph (b)(5) of this section shall be made by that operator of the aircraft on which the particular device is to be used. In the case of other aircraft, the determination may be made by the pilot in command or other operator of the aircraft.
As you can see, it does not say a) that this "determination" must be made before operating (IFR or otherwise), and b) it does not specify how this check must be done, so you can't say that such an such approach would not suffice, as regards compliance with the rules.
Lastly, please note that the above piece of legislation refers specifically to IFR flights, so it does not substantiate your assertion above that these gadgets are "prohibited from use in any aircraft, and particularly during IFR".
My reading of that is more like "if it's causing your plane to flip upside down during an IFR flight, then you may not be entirely legal to use it". But of course you are free to switch off all your electronic gear (including your and your passengers digital wristwatches, I assume?) before flight anyway... even on a plane like the one I fly, which hardly has a radio and a compass.
