If I were to carbonate at 30 psi for 2 days I assume once the co2 has dissolved the co2 tank continues to push a pressure of 30 psi onto the beer unless reduced.
If you do reduce the pressure prior to purging the valve or pouring a beer does the head space pressure drop to the new setting or does it remain at 30 psi?
If you reduce pressure at the regulator, the head space pressure remains the same until you release pressure inside the keg (pouring or purging). EDIT: It is also possible that the initial pressure (30) could dissolve into the liquid enough that the pressure in the keg reduces to your new setting. This would mean that you were not fully carbed at that pressure or that you lowered the temp of the beer.
Will the co2 tank only actually put more gas into the keg once the pressure drops at or below the new co 2 pressure setting?
Does the excess foam come more from too much carbonation suspended in the beer, too high a head space pressure, or some combination- or are these in the end the exact same thing?
Not exactly the same thing, but they are directly related. The amount of carbonation will be a factor of both the pressure in the keg and the temperature (colder liquid will absorb more CO2). If the beer is at the right pressure for the carbonation you want when serving at 40 degrees, it will be overcarbed/foamy if you serve at 50 degrees.
Excess foam will also come from too short of a serving line. A standard recommendation is to start with a long line and cut it back until you get the pour you want.
Because I'm lazy, the way I typically carb and serve from my kegs is to pressurize them to 30psi and shake for a while. I'll do this a couple times before I put them aside. Once they go in the fridge, I chill them and tap them. The first pour is usually crazy foamy, so I'll release head pressure to get a good pour. I close the fridge and come back when I want more beer. The keg has reached a new equilibrium and either pours well or needs to be bled again. When the pour gets too slow, I add gas at approx 10 psi to get a good pour and then disconnect. I do not have my kegs on gas all the time. It's usually pretty easy for me to get the kegs to the right pressure for a good pour with a nice head and good carbonation, but I'm used to my system after using it for years. YMMV.