According to the Gast 0823 pump curve, it's suggested max pressure is 10 psi...

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I know they will handle more, but I think your right...something is wrong. My Gast 0523 also has a 10 psi max and I have pushed it to 20 psi once to perform amperage tests. It starts fluttering and making obviously bad noises. If your diffusers all look to be operating well, I would suspect a bad pressure gage. Can you cross check the pressure gage, maybe swap gages from your other system? What is the pressure range of the actual gage. Many wide range gages are not very accurate at the low end. A 0-30 psi gage would be a good choice and liquid filled would be good to better handle the rapid pressure fluctuations from the pump.

I don't see anything obvious with your design unless your gate valves are causing restrictions, but your diffusers would be performing poorly I would think.

Your pump house must be very close to the same elevation as the pond surface for water to flow back up the mainline. It sure seems odd that a diffuser (even one without a check valve) would allow enough backwards flow to push it very far uphill. That is very odd because the actual slitted diaphragm acts a lot like a check valve...certainly a flow restriction.

Fish on!,
Noel