I know how you feel about cravings that take too long. When I was pregnant with my son I craved Perkins salad quite often....bad part about that was that the nearest Perkins is over an hour away. I tried to substitute other salads, but they just weren't the same. I also had the problem of having a craving and by the time I was done making said item I wasn't hungry for it anymore. (I think it was because I had to smell it the whole time I was cooking it.)
If you crave the same things often, you could always fix it and then store some of it in individual serving size containers in the fridge or freezer so that you could just grab one and heat it up.
I'm not sure about the cravings being nutritional needs or just being a weird thing. I've heard both ways. So not much help there. As for just having to have something, I would say eating the microwaveable stuff is probably fine in moderation.
I "tried" to snack on veggies, fruit, and yogurt when I had those starving moments after I just ate. Didn't always go that way though.
Sorry I'm not much more help. I hope some of the other girls might be able to give you some more advice.