J.B.
Hmmmm...are you concerned about the lack of crawling? Is she hitting other milestones? My youngest was very small, I don't think he hit 20 lbs until he was 18 months old but he was hitting all of his milestones (not early - he didn't walk until 15 month) and was staying on his own growth curve. It sounds like she's falling off of her own curve, which I would discuss further with your pediatrician.
It sounds like your daughter is getting some fat already in addition to breastmilk from the avacados and yogurt, assuming that the yogurt is whole milk. Butter isn't a bad addition to a child's diet - it's a real food, has vitamins and babies need saturated fats. I would add some organic butter and/or good quality olive oil to her veggies so that she is taking in extra fat and calories and I would introduce more carbs if she's interested in them - whole grain crackers, bread, etc. I think my youngest was fully on table food at about a year (in addition to breastmilk). It sounds like you're on the right track - don't fear butter and other good fats, but do follow up with your regular doc, especially if she's behind track for milestones. Good luck!