How can I reheat leftovers without a microwave?
We are in a similar situation, while the house gets remodelled we didn't want to buy another one.  The microwave we have is the combination microwave and exhaust fan model that's 10 years old, which can't be moved out without a lot of thought.  

But we find that it is a rather indispensable modern workhorse for reheating leftovers.  As long as the foods you are reheating have some kind of water/moisture it will reheat in the microwave very well.

This being said, we have gone to using a pot and some bamboo steamers on the stove top to reheat most if not all of our leftovers.  The steamer is pretty quick if you have an induction cooktop, which boils water extremely fast.  We can reheat several things at a time, by adding more steamer baskets.  The bamboo keeps excess water condensation from dripping on your food, so no need to really cover it with anything.  It reconstitutes dried out food more moist and it doesn't overheat it.  

But it doesn't work well when you want something crisp and warm. For this we use a convection toaster over, with the door slightly open.  We find that our particular oven gets really hot even with the convection fan running it tends to just burn things.  So having the door slightly open keeps the food from burning but still heats up.  

We might also heat up a pizza slice on a cast iron grill or pan with a lid or a sheet of aluminium foil on top to keep the heat in.  Also keeps any splatters inside.  

As for your particular foods, roast and potatoes.  I might try cutting it into smaller pieces and start with the steamer and then putting it in the toaster over just to crisp up the skin.  As for the potatoes I guess it would depend on how they are prepared.  But I think the combination of wet heat and dry heat will reheat nearly anything you got.  

But I do think that the microwave is a pretty ingenious way to reheat food.