... There can be a MEL on one of the engines bleed systems which then calls for the use of the APU to be on providing bleed air.
Uhhh..., definitely not.
1. The 747 is equipped with 4 engines.
2. One bleed system INOP (B747) does not require a alternate bleed source.
3. APU bleed is only available up to 15,000 feet.
4. The APU can be operated in flight up to 20,000 feet.
5. ...