Going dairy free and eating healthfully is definitely a go-er. I will be here to give you a hand! We did a gluten-free, dairy-free Christmas (actually it was also vegan) as my SIL is coeliac and DH and I are vegan, so I have hundreds of great recipes - the onion thing will be tricky, but I reckon I could work around it.
MummaB wrote a thread on going dairy free a while ago, and heaps of BB ladies had some great ideas on going dairy free, so there's lots of support right there! When I first went dairy free, I lost 4kg in 6 weeks, without doing much else (about 20km a week on the bike)
I think it's OK to have planned treats, as long as you get yourself into the mindset that you'll just have to do something to work it off. Especially when you can't have what's in front of you all the time, that must be really hard.
WW could be good to get you going, and it's pretty flexible around your requirements, I think. If you wanted to do it on your own, though, I think you'd do well by going with smaller portions of low-GI stuff, and maybe having a calorie counter book on hand for reference occasionally.
Every time I lose weight, I swear blind tat I won't put it back on, and every time, I put about two thirds back on. So slowly, I'm getting there! I think the important thing is to make sure you get back on the horse every time you slip up.
Make it a long-term plan - I think that's what I'm trying to say amid all this waffle! If it's sustainable, and you're not missing out on anything, I think you'll succeed permanently.
I'll leave the exercise bit to CQ - she's the pro.