Beer gas stout faucet co2 carbonation help!!

Alright, let me give you the basics. I have a 20# co2 tank with which i serve beer through my kegorator that i built using a traditional faucet. I recently swapped out a 5# co2 tank for a 5# beergas tank (75/25). I understand how to carbonate on co2, been doing it for two years. I also know that beers on beer gas should be carbonated to 1.2 volumes of co2. To carbonate 1.2 volumes i need to set the co2 regulator to 3 psi but a cornelius keg needs 10 psi or higher to keep an airtight seal. I dont want to use the beer gas to carb because it is a small tank. I read somewhere that un pressurized fermented beer has 1.2 volumes of co2 . Can i skip the carb on co2 and go to 30psi on beer gas for a few days. Is there a trick to co2 carb to 1.2 volumes of co2 in a corny

I am not having the exact problem but I am having difficulties getting a pour other than foam out of my stout faucet. I carbed with CO2 and then put it on beer gas. My beer lines are about 5-6 ft. Currently at 20psi, have tried up to 40 with no difference in the foam. Any ideas?

One more thing, I did have it on 12psi of CO2 for a month before switching to beer gas. Overcarbed maybe?