First let me say your daughter sounds wonderful!
The first issue you bring up is the sneaking the food, try putting the food into a higher cupboard where it will be harder for her to get, leave only the healthiest choices on a lower level, you are right you don't want to completly remove all of the "fun" foods this will only lead to problems down the road when she is older and can fend for herself. Remember some kids are just created bigger, it is ok, teach her to love her body no matter the size, but that she still has to be healthy.
Your next concern is socially, is this her concern or yours? Some kids just aren't very social, my oldest daughter isn't never was and she is fine with that, and my other daughter who is a social butterfly.Is her one friend good to her? maybe that is all she needs....one good friend, instead of a lot of ok friends. She sounds like a tomboy at heart and in time as she gets more into sports you will find that she will have more friends but friends who like the same things she does and have the same interests.
Third Q....puberty won't really start for a couple more years, you might be noticing breastbuds right know but full puberty doesn't start until about 2 years after that, I have 7 kids and know this first hand and from thier DR's. Sex and our bodies is always an open subject, but a lot of what they learned did come from the schools health program during the fifth grade, they were a little more able to do it so that the kids had a true understanding of what was going on with thier bodies, better then I ever could.
Relax a little, let her find her way, but support her and listen to her fears, as parents we try to fix it for them when sometimes we just need to step back and let them do it on thier own. Good luck.