I know that you can get evolves at as early as 1.5 stars, particularly w/ the first stage evolutions. It sounds like you got some sort of critical hit type boost, but I've never heard of anything like that happening before. It's the second > third stage or higher evolves that you have to wait til 2.5 stars.

If you try to feed something that is not classified as food, there is no message, it doesn't leave your inventory, and doesn't count towards your feed for the day.
As far as greens vs liquids.. try carrots. XD There's a lot of flavor-type mechanics built into rearing, so that might be the trick here. Like how certain wyrms like to give tea leaves since you evolve them with the Tea pop items, therefore they "like" tea. lol

Can't answer directly from experience on this one since I skipped it. ;x