The uncontrollables are fat because they stuff their faces all the time. Well, apart from Wilandra, who's pretty much the Atherton-version of Garron. I have no idea how she gains weight!

I've never had a Sim die on an outing, but I think it's technically possible. It's probably best to feed an ISBI family before going, and simply leaving anyone who don't eat at home. If they pass out on a community lot, they will wake up and get in the car when the group leaves, though. Needless to say, in TS3 they don't.