I don't know squat about spas, but I thought the overflow from the spa to the pool was something that could be turned on and off with a valve? I also would assume that as long at the overflow was running that the return pressure would be much less.
One final thought.. Why did you replace the guts to your DE filter? In my case they were way old and became very brittle. In fact the internal parts had broken and got into my return lines. I found a couple of small pieces in the pool and a couple larger ones behind one of the return eyeballs. Just something to keep in mind.
Sorry I couldn't be more help. If you get a chance post a couple of pics of your equipment pad, maybe someone else might see something of interest.
As a side note, I found that although I've got pretty good return flow into my pool returns, I could still stop the flow with my thumb. I had assumed I would not be able to do that, but it took a little effort, but not as much as I had thought it would. Like you, I have four returns.
Good Luck,
Jim R.