On #2 I'm not quite sure how you're divorcing "economical" from "cost per BTU". Economics is about $/BTU x efficiency, not just efficiency.
On #3, hell YES it will operate on both elements, at least some of the time, even if they're controlled separately. (Perhaps you meant that to read "A residential water heater with dual elements does not always use both heating elements at the same time?)
On #5, do the math. Only a few electric tanks will put out a max of over 4500W, which is 15,354 BTU/hr, at an efficiency of ~100%, delivering 15, 354 to the water. The 5500W electric heaters deliver 18,766 BTU/hr.
Atmospheric fired tanks have a steady-state efficiency of ~80%, and very few have burners with input BTUs smaller than 30,000 BTU/hr, which delivers (0.8 x 30,000=) 24,000BTU/hr to the water. That is a heat rate over 25% higher than a 5.5KW electric, and 60% higher than a 4.5KW electric.
Most 40 & 50 gallon gas-fired tanks these days have burners north of 35KBTU/hr, and recover in typically half the time of comparably sized electric tanks.
I don't know of ANY electric tanks that will recover as fast as the smallest-burner gas fired tanks, but maybe bein' in the biz and all you can point to some specific models where that would be the case?