Given butter contains trans fat and cholesterol (good for myelin though), is it advisable to include (10g/day) in 1.5yo normal-weighing infant's diet?

Yes: Butter contains a small amount of trans fat in the form of conjugated linoleic acid, which is good for you! not all trans fats are bad! cholesterol in butter does not significantly raise our cholesterol levels and cholesterol is not the cause of heart disease. No need to avoid butter, but far better if organic esp. From grass-fed cows. See my comment for some interesting links.