Yes that port is for the evaporative emission system for the fuel system. That is where you would hook a smoke machine to test for small leaks. However the onboard system tests for leaks as small as .020"! So if you have a leak it will code. no code=no leaks.
The other issue of a cloud of "smoke" I have only seen once and not on our trucks. I had a A/C system overpressure once on a E350 and the safety valve blew off and did exactlly what you describe. I dont know but that is my guess. If your system was overcharged that could happen. The cans for sale at the parts store are ok, but if you put in to much you can acctually hurt the performance of the A/C
The A/C system has nothing to do with the EVAP system so I would say they are not related.
On my 06 the fuel pressure test port is located on the fuel line at the rail. directlly above the alternator and right behind the lift bracket. I dont know what good it is because it looks like a pain in the

to get at.