On fittings issue:
NPT thread can seal properly against same size regular thread (including thread inside any flare fitting), while using a proper pipe sealant (Lock-Tite makes a special white stuff for gas connections).
The reverse is, of course, not true - flare fittings need flare fittings (of the same angle too) to seal together. You can use pipe sealant compound on their threads too, but it won't be very useful and will likely lock them forever.
I am not a big fan of NPT fittings myself but they do seal reasonably well with proper care. A lot of boat propane items require them - Trident regulators have straight thread fitting on the bottom that can only be sealed with a 3/8 NPT male, same goes for both inlet and outlet of Trident solenoid. They don't even provide fittings - you have to get your own (local Cawliflowers is the best source of those things, as I found).