Like everyone else, I hear what you are saying. it's not bad that you wanted a small dog, so don't feel guilty at all. A lot of people want a puppy that will grow up to be on the smaller end of the breed standard. The problem is that the "breeder" completely lied to you. Now, as a breeder, I know that adult weights are difficult to judge accurately, but we know when a dog will be big and when a dog will be small. It is rare that small parents throw huge pups.
Basically what I'm saying is that you were lied to, and I know you feel like you were taken advantage of.
But - your little man sounds so smart and so loving and so sweet! He will be everything you wanted in a compantion! And, like Mysweetbabies said, you can get another one down the road that will eb the size you are looking for - a companion for you and Oni!
I know that long-term medical care can be very difficult, but you have loads of support here! If you just need to vent, come on here and type. : ) We'll all listen! And, we'll all be praying for your daughter.