Re: Power
"I've always understood that the gel-filled batteries in UPSs are deep-discharge-tolerant"
As a general rule - no. Deep discharge cells are too expensive and unnecessary for the UPS use. Most UPS designs consider cell voltage below 12 volts as "empty" and will cut power off. Also, generic VRLA cell will rapidly deteriorate when its voltage drops below 10 volts. For that reason, typical UPS refuses to charge deeply discharged cells. There are exceptions, like solar energy storage solutions using a modified UPS and deep discharge cells. But charging circuit has to be well aware of the battery type.
https://en.m.wikipedia.org/wiki/VRLA_battery
AGM & Gel VRLA batteries:
Have shorter recharge time than flooded lead-acid.[15]
Cannot tolerate overcharging: overcharging leads to premature failure.[15]
Have shorter useful life, compared to properly maintained wet-cell battery.