Adding DE to a sand filter creates a very small DE filter by coating the top of the sand bed with DE. The usual recommendation for adding DE to help polish water in a sand filter is to add it 'til the pressure goes up 1psi - with 3psi rise, you had a denser DE layer that filled up quick.

With a centrifugal pump, the more water moved, the more load - less water, less load. The motor is cooled by the fan that blows over it. The pump is cooled by the water moving through it. I'd have some concern running a pump with zero flow that the water in the pump (no flow) would continue to heat up until it was too hot for the design of the pump components or it turned to steam either causing damage to the pump. I suspect that in your case (with the pump running on low and perhaps some flow) you're just fine.