I strongly agree that what you are dealing with is not about food. Your AP either is being thoughtless or she is being disrespectful, but either way, you have to have a conversation with her. I say this as a longtime HM who has had to talk to two APs over the years, including our current one, about their massive food consumption. In our current AP's case (and you can find the post about it from late Sept or Oct), AP was putting away 8 waffles in one sitting, 5 chicken breasts, 3 gallons of milk a week, two frozen pizzas, a quart of organic maple syryp in one week, 6-8 bananas a day, etc. In oue AP's case, his food consumption was also affecting his work: he was taking time away from interacting with my son to make himself enormous meals in the middle of the day.
So AP and I sat down and talked. I started by asking him a simple question: Is this the amount of food he was used to eating at home? He admitted it wasn't. What came out in that conversation were three things: 1) AP had never had free access to so much plentiful food. At home, food had been very restricted due to money and a very hard working mother who didn't stock the fridge the way our family fridge is stocked; 2) AP had decided - without mentioning this specifically to us - that he wanted to bulk up and gain weight during his year; and 3) AP was using eating as a way of filling time because he was feeling uncomfortable with being in charge with our son.
In other words, AP wasn't eating the amounts he was because he was hungry; he was eating for reasons that had nothing to do with hunger.
Once AP solved the third issue - interacting with our son - and became really good at his job, over-eating really slowed down. Yes, he still eats A TON, and we now buy waffles at Costco rather than Whole Foods and Aunt Jemima syrup for him rather than the good stuff, but overall, he isn't doing thoughtless eating like he was. And he now will say if we are down to four banaas, "May I go get some bananas with your CC today? We are low," rather than just finishing them and leaving us with none. But again - he is eating less because he isn't using eating as a crutch anymore, plus it has become more "normal" to have plenty of food around. Oh, and I told him that his weight gain wasn't my problem so he should get some whey protein powder to mix into his smoothies, which he did, and that helps a lot.
OP, I would address this head on. Again, this level of eating is either thoughtless - as our AP's was - or disrespectful, and either way, it isn't about the kind of "board" you are required to provide. NO ONE is required to provide an entire platter of brownies. Nor six sodas. Nor ANY alcohol at all.
So sit her down and talk. Don't mention the weight gain but find out how she is doing otherwise. Talk to her about whether she is happy with you and in the US. And then gently lead the discussion to food. You can begin with teh two easy ones - the alcohol and the brownies. Start by saying no alcohol period unless you specifically offer her, and then she only drinks the glass you or your partner pours for her. About hte brownies, I think this is a good way in to asking if she would do this at home. That will tell you something about her. If yes, then she is a huge eater and maybe is used to binging. So you can tell her that this won't fly in your household and it's a bad model for the children and from now on you will label. If no, then you can ask her why she is doing this here and see where that leads. But either way, it may be time to label.
In your shoes, I would stop buying sodas and gatorade. We are clear with our AP that the high protein bars and gatorade we buy are ONLY for our DC who is a competitive athlete, not for the rest of us.
Good luck with this one. Remember - this isn't about you limiting her food intake. It's about teaaching her to be respectful about the amounts of things she is eating.