Does such training and testing include the warning that approx 15% of fuel in main is/could be unusable if pumps are off?
Yes, it does. Not as a %, but as kg of fuel.
As does the Pilot Emergency and Malfunction Procedures checklist for either Transfer Pump (F PUMP AFT or F PUMP FWD) caution lights.
Further, as I am sure has been explained before on this thread - but I can't be naffed re-reading it all, there will be a FUEL caution light as supply tank qty falls and finally, as the qty falls even more, 2 big red warning lights (with a GONG) for LOW FUEL 1 and/or LOW FUEL 2.
On top of that the graphical display of the tanks will show the main tank not draining and the 2 supply tanks ebbing away.
Turning the Prime pumps ON at any stage of any of the above palava will technically do absolutely nothing to help the situation, let alone that turning them on is not a checklist item. The only time it is called for Prime Pump on is, aside from Start, is if FUEL PRESS (sys 1 or 2) caution lights come on.
Hope that answers your question. As I said earlier, I am not sure you could get a better system for letting you know when things aren't right.