Get a monitor app which includes a temperature readout (I like iStat but that is purely personal). Run lots of apps on the computer for half an hour or so without the case on and see how hot it gets. I set Chess to play itself and run an app called grapher (in utils) and select a couple of the 3D example, this seems to send my processors to around 100% which should heat up the unit. You guessed the next bit, fit the case and repeat. Does the system get any hotter with the case on or does the cooling work OK. If all good keep the case.
If I have this correct the card that the airport plugs into is an I/O card, I think it has all the external ports on it including the magsafe (ties in with may crash faster when on mains). Therefore the hardware chain is airport card into I/O card into logic board. Before spending anymore money I would try the following: -Reset PRAM - power on and then hold command-option-P-R wait until you have heard the boot chime a couple of times, it would also do not harm to reset the SMC pop battery out and unplug from mains and then hold power button down for 10 seconds, I am not sure if this ties in with airport or not but as magsafe and airport card are on the same board I am guessing.
-Retest to see if problem still present.
-Reinstall OS and apply all updates, at this point I would not install any other apps, not clear from above if you did or not.
-Next I would open up the unit and disconnect and reconnect all the cables relating to the hardware, so that would be the the airport card into the slot and ...