@ jorel
I'm sorry, I don't see your point.
Let me try this:
As per design and certification it is NOT possible to loose the AC/DC ESS Busses unless you have a AC/DC ESS BUS Fault.
If you lose all GENs, the AC/DC ESS Busses are powered by the batteries (and the Static Inv) until the RAT and thus the EMER GEN take over. An ALL GENs Fault AND the EMER GEN not working at the same time such unlikely, that it is not looked at during certification.
What happened during the Incidents on G-EUOB and G-EZAC was outside certification standards with double or even multple failures within one system and may lead to a modification once the cause for the fault is determinated by the investigation.
Regarding your questions:
1.) If the Stby-Horozon loses it's (only) power-supply it's indication is usless as hopefully indicated by a warning flag. The flag should show immediately and it won't take long until the gyros lose enough RPM for the indication to become erratic.
2.) I don't understand, do you mean FLIGHT ON BATTERIES only? One BAT is supplying DC ESS BUS and the other AC ESS BUS via the Static Inverter. What would you suggest?
3.) You mean an AC BUS1 Fault AND the ESS ALTN FEED does NOT work? Well once again you become a test pilot, as this condition is not mentioned in the books. It's an emergency an the Commander may do whatever is necessary in his opinion to bring the flight to a happy ending.
It MAY help, however you will lose AC BUS 2 as well when selecting EMER ELEC manually to ON. Greatly depends on the cirmcumstances if you ask me, if the weather is ok and you can land rather soon, I rather would not try.
Regards, MAX