It may already be an implicit assumption, but I think that time to cook is probably not as important as how to cook. While the power setting does affect the time to cook, the importance of time is to control how hot the parts of the food get.
As with all cooking, there is a need to control temperature. Running the microwave at less than full power allows time for the heat to be conducted from the hot parts to the cool parts. It is similar to searing vs. sauteeing.
I think that the accuracy of most microwave oven's power settings are good enough (my own testing shows about 10%). I would be more concerned about how I wanted the item cooked. Some examples:
- When I want to reheat things like a thick soup, I run the microwave at full power for a short period of time, stir, and run the microwave again. Stirring the concoction distributes the heat faster than the conduction process.
- When I want to defrost frozen foods (think meat), I use the lowest power setting. This prevents the edges of the food from getting cooked.
Sunflower lecithin (sorry, best link I could find) is gaining in popularity as an alternative to soy lecithin because it is widely perceived to have a neutral taste and actually has superior emulsifying properties. It's a little on the expensive side, though.
Soy lecithin by itself doesn't taste horrible if you buy it as a food additive (as opposed to a nutritional supplement). It's common to find in a lot of popular frozen pasta-and-sauce dishes, such as Michelina's, which obviously must be microwaveable without the sauce curdling or separating.
Another option is Sodium Stearoyl Lactylate, if you can get hold of it. This one adds a little sweetness which is generally pleasant-tasting, or at least inoffensive. You'll find it used most often as a dough conditioner or in baked goods, but another popular use for it is in commercial sauces, although unfortunately I can't find specific examples right now.
But probably the best emulsifier for what you're trying to do here is xanthan gum. It's also used as a gelling agent, but in small quantities works as just an emulsifier. You can find it, for example, in this Bertolli Creamy Alfredo Pasta Sauce - exactly the sort of thing you're making. It's also used in the Stouffer's Alfredo sauce.
Mix in a little lecithin or xanthan gum as an emulsifier while the sauce is fresh or cooking and it's very unlikely that your sauce will separate, either in the fridge or in the microwave.
As for what you can do to specifically prevent separation while reheating, if you didn't add any emulsifiers in the first place... I really don't think there's anything you can do. If you're a slow enough eater the sauce will eventually separate right in front of you; that's just what happens when you have water and fat in the same sauce. Best to reheat on the stove top in that case, and stir frequently to prevent any further separation.
Best Answer
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.