I run adaptive mode with a constant D5 NEXT speed ( 75% power ). When I increase the power to say 100%, the negative pressure and required negative pressure increase. When
I then set the power back to 75% the required negative presure decreases to the prior 75% power level and the actual negative pressure remains the same.

What's going on ? I suspect the design of the Leakshield pump and valve allow for incremental increases to the negative pressure but not incremental decreases
to the negative pressure. There is a full release of the negative pressure when the Leakshield mode is set to "Release".

Hopefully someone who actually knows can explain it better.

LEAKSHIELD will never reduce the negative pressure in the adaptive mode as this would cause earlier pump action. The goal is to use the vacuum pump as seldom as possible. It's like wasting energy.
So after a time the pressure will decrease by itself due minimum leaks. You can reduce the pressure by the setting the maximum pressure value in the settings.

if the pressure (or basically the reservoir level) drops very slightly when accelerating the pump, it's likely you still have air trapped in the loop somewhere too.

It alows the LEAKSHIELD to use a lower negative pressure in your system calculated by the pump/flow parameters. But this lower pressure is not forced by the LEAKSHIELD due opening the release valve. In the case the pressure will drop the LEAKSHIELD will only use the vacuum pump to achieve the adaptivepressure values.

Is it really worth it to tie the pump speed and flow rate to coolant temp?

There are endless debates and a bunch of YouTube videos about how flow rate affects loop heat transfer efficiency. There are many factors including laminar flow versus turbulent flow, coolant temp variations at different points in the loop. coolant friction, heat transfer coefficient, and pump waste heat (which for a D5 is transferred to the coolant). Increasing the flow rate will increase turbulence in a CPU block, which improves heat transfer, but it also increases coolant friction and the amount of pump waste heat being transferred to the coolant.

Some feel it's best to find the optimal minimum pump speed and leave it there. To do this, set the pump speed to 100%, let temps stabilize, and then start decreasing the speed in 5% steps. At some point, temps will start to rise, typically around 30 l/hr. Increase the pump speed 10% above that point. This should be a bit above the minimum speed required for the loop to function properly. Lower pump speed means less noise and less pump waste heat passed to the coolant. Its also easier on the pump, and you would not have these pressure fluctuations with the Leakshield.