I think it works fine. Obviously it's a device to make it so Gohan HAS to get sucked off for them to find Babidi's ship, but Spopovich's lifelessness was a natural enough eccentricity to tip the characters off with considering what he is.
Damage fills it up. If Vegeta wrecked SSJ2 Gohan in 2 seconds (think Android 17 vs Trunks), he'd fill it up just as fast. This could, conceivably, also happen to a SSJ Goku that's stronger than SSJ2 Gohan.
Vegeta is (probably) talking about when they're in the same form when he says "Our powers...
Yeah, not super commonly, but commonly enough for it to be notable. What do you mean "How does that work?"? It just does.
HxH, FMA, Hellsing, Shaman King, Fruits Basket, and Evangelion are some examples of anime that have been rebooted.
Ki reserves = 10,000
Stamina = 100%
Full power = 1000
Goku uses a casual KHH
KHH = 3000
Stamina remains unchanged
Ki reserves = 7000 (or a bit less)
Stamina = 100%
Full power = 1000
Goku fights and then uses a full power KHH
KHH = 5000
He gets tired
Ki reserves = 2000 (or less)
Stamina...