I made mine from block so not on the same level as shotcrete, but I didn't use anything for the pipe
penetrations but the pipe. I just packed hydraulic cement around the perimeter to seal it.
Personally I'd put the all the returns you have drawn in and maybe even a couple more, like one for
the shelf that someone mentioned will collect debris. Just put a ball valve on each return and turn off
any you don't use. If you find your shower flow is too much for just the 3 and the waterfall return, you'll
be glad you have some more. Or you can alternate some you don't really need all the time but can
activate if you see debris build up in an area with no flow. It's simple to add them now, but hard to
add them later.
Like Grumpy, I used 3/4" pvc on my pond with bottom drains for the air lines and I just direct buried
everything.
I think your filter design will work well. Might need something for fines with just a sieve and shower... like
a sand and gravel filter, but much better than the sieve and bead imho.