I'm not an Rf expert, but...
What was the RX in the crashed plane? The RX has more to do with getting "hit" from other radios coming on than anything.
Unless the RX was a rather old single conversion, or AM RX, I'd say the chances of the radio casusing the crash are basically near-0. Espeically since the model was fairly close to it's TX, and specifically closer to it's TX than any other TX (if I understood correctly).
Most of these IM effects are worse when the plane flys away from it's TX and close to the interferring TX's like on a long low pass down the runway with the pilot at one end and the other TX's at the other end. That's your worst-case situation for interference of all types.
(My GWS park flyer RX (single conversion, FM) on channel 27 gets really unhappy when some other TXs come on, espeically if they are on channel 50, even with their antenna's down. While all the JR, Hitec, and FMA duel conversion FM receivers I have routinely handle much worse when it comes to multiple radios on at once. (I flew a combat contest two weekends ago (Paris, TX) where we had as many as 13 planes up at once, with pilots standing all over, sometimes closely spaced. I didn't see a single radio hit, or hear anyone complain about interference. Our modern gear is amazingly good at rejecting all kinds of garbage).
A possible cause for the crash could be a mis-labeled crystal on one of the TX's in question. I've heard of this before, though it's not common.
Sounds to me like someone dumb-tumbed his plane, and is looking for an excuse
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