I definitely wouldn't eat more than I felt comfortable eating, just for my own health. If taking leftovers back to her room is an option (sounds like it is), maybe make that the plan--either in your own head, or aloud with her if you think that would help. "This steak will be great tomorrow morning with some eggs for breakfast--DH and I can split it!" I understand her wanting to get her money's worth, but really, I'm not going to make myself sick for that. She presumably knew going into the situation that those were the rules.
One approach you could try is asking her directly, before dinner, what she would like you to do. "You've expressed several times that you don't think DH and I eat enough for what dinner costs. We simply aren't able to eat more, so how else can we help to alleviate that?" Suggestions might be giving her money to pay for your meals, bringing leftovers back to eat for a later meal, only one of you accompanying her to the dining room so she only has to pay for one guest, etc.. Maybe some of them will seem unpalatable enough to her that she will realize paying to have you both at dinner is the option she prefers. Then whenever she comments you could say, "Oh, did you want us to do Suggestion X instead, like we'd discussed? No? [implied: then quit complaining about it]"
It might be worth a try anyway, although it sounds like at her age and personality it would be rather hard to get her to change her habits. I might still have that conversation with her once, so that everyone acknowledges there ARE options, then I would feel free to ignore her food pushing or money comments after that. Just because someone says something, or wants you to do something, doesn't mean you must respond or do it.
Personally I don't really like the idea of secretly leaving people money. In some cases it works perfectly well, don't get me wrong; but generally it has a bit of a patronizing whiff to me, with someone who is cognizant enough to make their own financial decisions. At most I would keep the money in my possession but bank it in case they needed it "back" someday. Also I think it could be rather confusing for an older person to suddenly find cash, especially a fair bit, in their pocket or a drawer or something--it might cause them to worry that they were becoming mentally unreliable, because they don't remember where the money came from.