I really wouldn't stress her weight since she isn't loosing weight. My son was 20lbs at 9 months and now at 2 1/2 yrs is only 30lbs! So how is that for some weight gain! There were some office visits where he hadn't changed but maybe and ounce or so on the scale and his Ped was never worried about it. My daughter was 20lbs at 1yr old and she is 8 1/2 and weighs 50lbs! She is thin built just like I was as a child. As long as your daughter is active and healthy, don't make it too much an issue.
As far as her diet goes, the instant breakfast isn't too bad for her but it's mostly empty calories with all the sugar in it. If that is the junkiest thing you feed her, then great! You might want to increase her juice intake though, 25 juice/75 water. Yes, there is sugar in juice, but it is more natural than added as long as you are using 100% juice. For foods, break the habit of throwing food on the floor. Remove her from the table when she does this. After a min, put her back with a fresh plate of food and try again. If she throws the food, she is done with the meal totally. To her, it's more fun to get a rise out of mom than eating my dinner! She'll get the hint and don't worry about her going hungry, she won't. And after she realizes that this is no longer tollerated, she might be more inclined to make a new game of eating than tossing the food! Also, don't over load her plate with food either. Just a few bites of everything being offered will do. Sometimes, kids think they have to eat everything we put on their plates which turns them off to eating as well.
For good snacks, mulit grain cheereios, life cereal, yogurt, fig newtons, soft fruit grain bars, etc.