How critical is "tow ball height"?

Nose-down or level is recommended primarily to prevent sway.  Nose-up induces sway.  My popup was a sway death trap and I had to keep it very nose-down to help.

That said, I have no choice with my Suburban.  I bought the longest shank they had for the E2 and I'm still slightly nose-up.  I do have the risers.   When it is time for new tires in the next year or two I'll look into tires with a larger outer circumference to try to gain a little more height.

The refrigerator only needs to be horizontal when the Pod is at rest.  Towing rocks the evaporative system enough so that the frig runs without a problem.  (If you adjust the Pod to be horizontal while towing...the refrigerant is constantly being sloshed around anyway).  So the best advice is a slight downward tilt to the front to help with sway control.

Once you get the trailer hooked up you will probably find that it is nose down. I have more than an inch of sag in my tv even with the WDH on.

No, not necessarily, but if your trailer is nose up some, and/or your tow rig is marginal, then this will really lighten up your hitch weight with the added weight behind the trailer axle. In my case, I had just traded trailers, was nose up, hadn't dialed in my WDH yet, and had a partially full water tank behind the trailer axle. The state  patrolman who handled the wreck was a trailer guy, quizzed me, and said it was a "Perfect Storm" type deal with several things factoring in--inadequate WDH set up, low hitch weight due to added weight behind the axle, water slosh in that tank, short wheelbase truck (even though a fullsized P/U), AND the wind gust was so instant and severe that control was immediately lost. I had my choice of a 200' drop on the left and a rock wall on the right.  I wheeled it over and chose the wall. No injuries.  So a Prodigy, WDH, and sway control weren't enough to overcome Mother Nature and my ineptitude .