Re: Lots of questions!
Welcome to the forum!
To answer your first question, the's no real "should" when it comes to how often a 15 month old nurses. By 15 months babies are all over the map when it comes to nursing frequency. Some nurse just once a day, others nurse as frequently as newborns. However, I have often heard it repeated (and repeated it myself) that as long as a toddler nurses 3-5 times a day, you do not need to supplement with animal milk, because all baby's needs for "dairy" are being met.
To answer your second question, milk doesn't naturally dwindle as nature's way of weaning. Milk production is maintained by demand. If baby demands a lot and drains the breast thoroughly, she will maintain plenty of production. If she doesn't nurse well, which happens when a baby starts to lose interest in nursing or when she gets lots of supplemental bottles, production will diminish. So why don't your breasts just get bigger and bigger with time? Because a baby's need for milk peaks in infancy, when her diet is 100% breastmilk. As she transitions to toddlerhood, and starts eating solids and drinking things other than milk, her need for milk actually lessens.
You can certainly meet your child's needs for the fat and calcium that comes from dairy by using other foods to complement your milk. Avocado, for example, is an excellent source of healthy fat, and things like tofu and leafy green vegetables are rich in calcium. Offer those in conjunction with nursing, and you probably don't have to worry about your baby's "dairy" intake- particularly if you nurse 3-5 times a day or more often. This is why I nurse my kids until 2 at a minimum- as long as they are nursing several times day I don't need to worry if all they eat on a given day is crackers and dirt off the floor.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"