My grandparents in Toronto eat little, but still desire baking or broiling. Is it normal, and would they be wasting money and electricity and the environment, to use just one oven rack when oven racks still have space?
At any time of year when heating the home is desirable, the waste heat isn't really wasted. In Toronto that will be quite a lot of the year.
Most of the energy used in baking is in getting the oven hot in the first place, so sequential baking is almost as good as simultaneous baking. Modern ovens hold their heat quite well so even baking again a couple of hours later is better than the next day.
Of course cooking extra food isn't going to reduce waste, but cooking future meals - things that can be eaten cold, or easily reheated in a microwave - will use less overall. I make individual pasta bakes in dishes that will go in the oven, freezer and microwave, for example. I also try to plan on baking bread in the afternoon when I'll be cooking something else in the oven for dinner, but that plan normally seems to fail.