I've encountered some of the same challenges you have mentioned. I asked similar questions to our pediatrician a few weeks ago.
I think you'll feel much better if you read "Touchpoints," by Dr. Terri Brazelton. I bought it this week and it helped relieve a lot of stress. He writes about picky eaters and offers minimum daily requirements when you think your baby isn't getting enough nutrition. Here is brief sketch of the minimum RDAs:
1. Whole/Breast Milk - 16 oz
2. Protein - 3 oz (Hamburger, eggs, beans)
3. Iron - Usually in meat or or beans
4. Orange Juice - 1 oz OR a few bite sized pieces of fruit
5. 1/2 slice whole grain bread
6. 1/2 cup whole grain cereal
7. Cooked green vegetables - whatever they will eat, usually green beans
Brazelton explains that a toddler's diet must not be assessed in a 24 hour period, but rather a weekly average, since eating habits change so frequently. I recommend keeping track of what your lo is eating over a week or so. You just may find that on average, it's quite close to the RDAs, which, according to Brazelton, are just fine for sustaining your toddlers growth and development.
~ Relaxed for now.