The drain can be lifted a 1/2 inch once glued. Is this too high/too low? Should I have something besides a 45 at the end?
Do you put a air line hose in the hole in cap? Or just leave the hole?
Will this help reduce splashing? Should I do this in my drain on the skimmer side also, just without the 90 at the bottom of the Tee? Should I cap the Tee with a hole like a durso or is this sufficient?
Well it looks fine to me but you dont really need all those unions IMO. One union at the pump and one each on the bulkheads from the tank should be fine.
The drain looks just about right. Dont glue on the last 45 in case you have to shorten the pipe up some.
I just have holes in the caps on my dursos again dont glue on the cap because at sometime you will have to take the cap off to clean the salt creep.
On your return line you probably dont need all those 90s and tee one 90 right at / just below the water surface would probably work just as well.
This is JMO HTH
Start with a 1/8 hole and go from there. If the hole isnt big enough the water in the overflow will move up and down. When its right water will stay level. You shouldnt be able to hear much if anything at all if its set up right.