Your tanks looking really nice Sean.
If I could do it again I would raise the backup about a quarter inch so that it's very slightly higher, but it works very well as is.
The only thing that's difficult to quiet down is the weir with the new MP10, every time it goes from surge to wave it takes a couple of minutes before the water level in the inner box stabilizes and you stop getting that falling water noise over the weir.
Sure makes a check valve seem like a dumb idea.
Not an issue. Just add an elbow inside.
If that's too high or too long to fit...
Snug the elbow into a length of PVC, slice off at least half of the would be otherwise cemented joint.
If you have to run both ends in a power saw, make the 1st cut the longest for purchase on the 2nd cut.
Now, is the bulkhead slip on the inside or threaded?
If threaded, 1st screw on a conduit nut that fits before cutting length down
and don't cut as much off since the threads are tapered I believe.
Then when you remove the nut, it fixes the screwed up threads somewhat
and a couple wraps of Teflon tape will facilitate a more smooth task.
You should try lowering the MP10 to accommodate your weirs operation or noise.
Check valves are dumb. They foul so they fail.