Interestingly an all-engine failure in the 747 would mean you have to descend to an altitude that permits the APU to be started since the 747-400 does not have a RAT.
Nope. The APU can't be started while airborne (prohibited in the Classic, and I believe inhibited as well in the -400).
Emergency electrical power (batteries) can be used to restart the engines; there are inverters to provide essential AC power. Hydraulic power is sufficient with windmilling engines down to 160 KIAS.