RE: How close can transmitters be
You are correct. Channels 20 and 32 could get a 'hit', but chances are not particularly likely with modern systems and the way most people fly...but the scenario I mentioned with the 3rd flyer 'way across the park and his plane near you two fiers...well then the chances of the 3rd party's model getting 'hit' does become greater.
Please remember, though, that the 3rd-order intermodulation result is a very weak signal when compared to the transmitter's normal signal, and modern receivers have been made with the awareness of intermodulation interference. That spectre came up years ago when the current channels were set up. A LOT of research and development went into making sure that 3rd-order intermodulation was minimized as a problem source. If it wasn't, there would be a LOT of problems at a LOT of flying fields, contests, and so on. R/C car racers are also affected..
Please note that the formulas work only because the channel numbers are evenly-spaced. You should really work this out with the actual frequencies in the formulas. That will give you the correct answer, but using channel numbers works just as well for our purposes.
One last thing. If one of the channels you come up with is higher than channel 60, it won't affect the surface users. The formula works with their numbers, but they're on 75 MHz...no way a 3rd-order signal from 72 MHz transmitters could affect them. The reverse is also true.