In order:Ok, well, yes you've basically done exactly what I had in my head. A huge tank serving a variety of different needs, from pvc nipples to auto waterers.
Could a stock tank heater keep the main water warm enough to keep nipples from freezing?
Is that an automatic dog waterer? I basically want to accomplish that, but with a larger tank for larger animals. I think I have a float valve in my garage somewhere I can use.
I didn't realize polytote already had that nice outlet at the bottom.
Do you recommend raising the tote higher than ground level for increased water pressure?

I don't know. We don't get cold. Ok, we were sub-Freezing for 28 hours continuous this year (new record) and the top iced. So did the water line. Nothing busted. I'm confidant a stock tank heater would keep the tank itself from freezing thru in all but very extreme temps. I am NOT confident it would keep the PVC with the nipples from freezing - you would be better off using electric freeze tape for that.
Yes, it IS an automatic dog tank waterer. I have stock tank floats attached to the one on the barn, both work fine.

NOPE. Elevating the tank adds less than 1/2 psi per foot in height. The one for the barn adds almost 1.3 psi at 3' in elevation. However, elevating the tank DOES make it much easier to service, when you have to. If you had nothing but birds, I'd recommend just the 8" or so lift I have by the goat shed, sitting on CMUs. If you had goats or horses, or similar where you would want a trough several feet off the ground, the base of the polytote should be the same elevation or higher than the intended trough (and you can still run a lower line to proovide nipples/cups, etc for other animals.