It looks Good so far. Your reasoning sounds on the right path.
I am in Houston, TX and we all do things a bit different depending on the region.
Personally, I would not use the strainer, unless you have a problem with your water.
The unions is a preference I guess, It is real easy to cut the device out and put in two couplings when you need to come back in with it.
The drain or hose bibs can, if placed before the backflow device can make the backflow device ineffective. After all the device is to come before any potential to connect to water cross connection. So if you add one, I don't recommend, put it on the discharge side of the PVB.
And Remember to get that bugger tested by a pro.
It really does need to be tested.
You would not believe how many brand new devices fail because of shavings of PVC or teflon or rocks in the line.