So there's no issues with hot soaked restarts of an EFI engine?
Not in my experience, and I'm using dead ended fuel rails.
2. We have a pair of Walbro GSL393 EFI pumps, suitable for in or out of tank mounting though I am curious how they were tank mounted.
Beats me....
3. Check valves. I thought that these pumps would not free flow either direction if shut off thereby not needing check valves?
Don't think they are necessary, I've never used them.
4. If an EFI pump were mounted in a sump of approximately 1.5 qt capacity, this fed by a low volume/low pressure Facet pump from the main tank, between the heat generated by the pump itself and picked up in the circuit, would the flow of the facet pump replenishing used fuel with the excess returning to the main tank keep the pump, sump and circuit fuel at a reasonable temp? What would the minimum volume be?
Keep in mind that the circuit fuel is mixing with the replenished fuel so some heat is going as overflow back to the main tanks.
Sumps can work but frequently require fixes for their drawbacks. If you don't have a really good reason for needing one, why use one?