As for cooking them, I find they're tastier and the sauce is tastier, if the meatballs are browned in the pan you will use for the sauce, set aside, and then the garlic and onion slowly sauteed in the fat, and then the pan deglazed with the tomato, or first with wine if you have some, till it's reduced, picking up all the sticky meat residue, but i rarely have wine hanging around for cooking. You don;t have to completely cook the meatballs, just slightly brown them, then when the sauce is going, put back into the sauce to finish cooking. Their juices flavor the sauce and the sauce flavors the meatballs.
I'm really curious where "spaghetti and meatballs" comes from, though. No one i ever met in italy who hasn't spent time abroad knows about the dish (served as we know it), or if they do, they know it as an american dish. Yet, i'm sure it must have been italians who brought it over - perhaps it's a very local dish typical of some town.
Here my mother in law usually makes meatballs like small meatloaves, about as big as a small orange but oblong, browns them in the pan and then adds wine or brandy. No tomato. Sometimes she makes them bigger and puts a whole hard-boiled egg inside, and then slices them to serve them. But always as a second course, always after the pasta course. For her it would be very strange indeed to serve the meat WITH the pasta, unless in a composite dish like timballo or lasagne, and there wouldn;t be much meat in it, since it would be followed in any case by a main dish.
She makes very tiny meatballs about as big as dimes or at most nickels in diameter, browns them and puts them in baked pasta dishes, typically "timballo di pasta" along with tomato sauce, hard boiled eggs, peas, tons of parmigiano and an egg to hold it all together so it can be baked, slightly cooled, then unmolded and sliced. (Personally i don;t like it, but it's very popular in central italy). In some regions they put tiny meatballs in lasagne. It would be only for a special sunday dinner or holiday. But i have never come across meatballs in sauce with spaghetti. Anyone who knows, I'd love to find out where they come from. It certainly is a very satisfying dish.