You can clean regulators but they tend to get ‘soft’ with age. You will see this as pressure fluctuation when taking shower and washing machine starts. Properly functioning regulator with a good supply ( ie. not a old house with a undersized supply line) will not notice flow fluctuations.

The larger reg ($126)is what you want as it has higher flow capacity. Watts is good brand. Regs tend to be ok wrt long term reliability.

#1 Yes, the horizontal section closest to house. Heat the elbows only not the pipe. Tape several layers of aluminum foil to the house with a cutout for the pipe. It will help protect the house paint from your torch. Aim the flame away from the building.

Heat will conduct into the wall through the pipe. Be wary of fire. Bucket of water is a good idea. Be quick with the torch and if you have access wrap a wet rag on the other side of the pipe. ( Looks like it goes into an enclosed wall so can’t be done. )

Ball valves are non-directional but check where the handle position is when open. Looks like you may be close to the ground and you need it to fully open.

When you pre-assemble everything, be sure to paint flux everwhere that will be soldered. Inside slip joints as well as the male ends and solder when installed in place.