Bro, If the emitters will not be covered by soil then no big deal. The amount of water in that one drip line is insignificant and a very small risk to your mainline anti-siphon valve or water supply. If it makes you feel better install it just after the valve. Aloha

If you put the compressor inside, it won't blow air/water out because of the back flow preventer (that only allows water/air to come in). I would blow out the whole system as has been done in the past and then open the bleed screw on the ball valve inside and then blow a little more (all the blowing should be from the outside). This last step should clear any water that is between the back flow preventer and the ball valve. If you don't have a bleed screw on the ball valve, you are going to have to install a vale that has one or you will have to somehow disconnect the pipe in the cellar each year. It all comes down to finding some method of getting that last bit of water out that is between the BFP and valve in the cellar.

It can be serviced but before you go off calling a plumber give something a try. Now I don't know what model you have so I am just giving a broad idea of what to do. On the top of the valve there should be 3-4 bolts or screws that can be taken out. Shut the water off on both sides and take the top off of the valve. There should be a whole cartridge that comes out along with some seals. Just take that out and rinse it off real well. A lot of times you will get some dirt on the seals and it won't allow it to close all the way. Once you rinse it off you can put it all back together and turn the water back on. We have our valves at work leak at least once a year and 90% of the time the above fixes it. If that still doesn't do it you can search for a brand new cartridge or call the dreaded plumber. Hope that helps. Feel free to ask any more questions. Matt

If there is a valve on the supply between the BFP device and the auto leveler try closing it 1/2 way. This may slow the filler and reduce the water hammer you are experiencing.