Finished the plumbing up last night/this morning. Decided to do a fresh water leak test.
Started to fill the sump, then turned on the return pump. Nothing but Er-04. Apparently that means the impeller isn't spinning.
After a short freak-out, then posting a new thread, Chris A. told me to wait a bit and try again. 5-10 minutes later, after Googling (and calming down), I filled the kitchen sink for another test. Success! The pump was working. Not sure why it did that, but I guess that's par for the course with these pumps.
Forgot to open the return line ball valve at first so I was worried something wasn't working. Once I realized that water started immediate flowing to the DT!
DaughterX and SonX assisted while we filled the tank from an outside hose. They were looking for leaks, watching levels, etc.
After getting everything filled (DT/Sump) I had DaughterX unplug the pump and we "tested" a power outage scenario. Plenty of room in the sump, even with some extra water in it.
We also tested pulling the main drain standpipe out and the water levels were still fine. While I had it pulled out I took the strainer screen off the standpipe (I think it was causing suction to occur, making more noise).
Also took the strainer off the emergency standpipe while I had it out.
I did add another straight piece of PVC to the main drain (near the filter cup holder) so it's under the water level like Chris A. pointed out the other day.
I think everything is settled now. Got the return pump dialed all the way down, and the main return line ball valve closed just slightly; this keeps the splash noise level down to almost nothing. I think once things slime up the noise will lessen even more.
Fingers crossed, no leaks appear!