You use purge gas while brazing to avoid oxidizing the inside of the pipes. Copper likes to tarnish when it gets hot. If there's no oxygen, it can't tarnish and stays clean. The copper smut on the inside of the tube will flake off and clog up your system if you try to take a shortcut. I use propane as a purge gas, and it works well, as long as you discharge it away from where you are brazing. As always, proper active ventilation is essential when dealing with anything flammable.
As to your question about mechanical fittings, I like flare fittings second to brazed. They also take a little practice to do right the first time. The thing I like about flare fittings is they are not expensive, and they are readily available at hardware and plumbing stores.
I have seen compression fittings used in automotive applications, but they crimp the ferrule to the tube with a nut behind it, just like flare fittings. The fittings used are specialty fittings made just for auto a/c and are much more heavy duty than standard plumbing compression fittings. They aren't cheap, either.
Yes, you can manually reverse the flow with hand valves, but once you source the valves that will handle high pressure refrigerant and the tees and such to connect them all, you've spent more than the price of an ebay reversing valve. Also, in heating mode, the evaporator has a tendency to freeze up while noone is watching. A defrost control is a lifesaver during chilly weather.
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