Can a baking stone compensate for my oven's lack of a top heating element?

all gas ovens only heat from the bottom and baking works just fine. Your oven may not be getting hot enough as the thermostat may be broken.

If you have no hob, investigate induction hot plates. I use mine frequently, sometimes with a cast iron skillet. Less energy than any oven solution, and not that expensive.

For a pizza yes but not really other things that need equal cooking on top. I guess you can flip them and try.

Probably so, IF you heat the stone really well ahead of time. But keep in mind that air flow may be restricted to some extent.

It may restrict some of the natural convection of air flow.

Does your oven have a broiler element? If so turn that on for a bit to get the top of things browned.

If your oven has a convection setting try that as well it will bring the heated air around and will allow things to brown better. If it does have a convection keep in mind that the oven will probably compensate and the temperature will be about 25 degrees F. less

I would give it a try, you can heat the baking stone hot enough. But why not just put your (metal) dish on top of the baking stone, and turn the potatoes over halfway through baking?

It can't hurt to try, but I doubt if it will make much difference, if any. Some aluminum foil would do a better job of reflecting heat back down, though.