I have done this, too, using mountain mode before going below the threshold to preserve enough charge for driving electrically at the destination without preserving too much. However, using something similar to hold mode with a larger battery buffer doesn't seem to be what "Volting" is about. He speaks of using mountain mode to recharge the battery, which not only loses energy in the conversion from kinetic energy to electric and back, but adds the losses of converting electric to chemical energy and back, along with additional heat loss, and, other than in exceptional cases, prevents a direct mechanical link from the engine to the wheels when recharging the battery.Yes it can. It is less likely to do it at high power demands than simple 'HOLD', but if the SOC is not too low the engine runs at a modest power setting and will run just like hold, albeit at the next power set point up to assist in battery charging.
It is not so dumb that it simply runs flat out as fast as it can, it eases off as it increases the SOC.
The engine may make up some of the difference by running at a more efficient engine speed, but I doubt that it makes up the whole difference in every case, much less making up more than the difference. I also believe GM's engineering team when it says that power split is 15% more efficient, because it just makes sense.