The main reason for not putting the valves inside is exactly what you posted, the abundance of holes and pipes you would have to run from the garage/basement to outside. This is sometimes common with commercial properties that will have the valves mounted inside a designated maintenance area or in a shed.
Outside the valves can be mounted below grade or above. I have seen mounted above ground many times when down south or out west. I have never seen a professionally installed system up here with the valves above grade. If installed properly the wires will be perfectly safe from water and weather. For example I sodder the wires and heat shrink wrap them. Also we install the valve box on about 8" of 3/4 stone to help with drainage.
The PVB should be mounted outside due the spurting off you described. If you want to mount your backflow indoors you can use a double check valve, these can be mounted in doors as the don't "spurt off" and also can be mounted below the grade of your highest head.
Another reason not to mount the valve manifold inside is simply due to the amount of pipe. On a larger system say 9 sones. I might split it up into 3 different valve boxes. For the three furthest zones I will run one long pipe to that far box and then manifold it off for those 3 far zones. If all of the manifolds were inside I may have to run a series of pipe 200'+ to some of the zones, instead of just running one main line back there.
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