You don't even need to use gas to purge the back of an open corner weld.. Just put a piece of non ferrous material clamped tight on the other side and giver.. I do it all the time
The best way to fittings like that is to use a piece of aluminum foil and tape it onto the backside of the fitting so it has at least an inch gap between it and the fitting (so you don't burn through the foil) and poke some holes in the foil for the argon to flow out then purge through the other...