Are we required to eat every meal and every food/drink under the sukkah?
What if it is raining? Cold? What if there are too many people to all fit under the sukkah (we have lots of guests)?
Those are just some examples
Generally speaking, treat the sukkah as your dining room. If you had a massive leak in your dining room roof, you would eat elsewhere (potentially even going to a neighbor's house).
There is a stricter standard for eating one piece of bread on the first night of sukkos, but otherwise:
If there are too many people, "crowded" per se isn't a way out. However, we would apply a priority scheme for who should be in the sukkah, starting with those who must and ending with those for whom it's meritorious. The only ones who MUST be are adult (age 13+) males, eating bread or possibly other grain products. So the Atkins people should stay inside if there's really no room in the sukkah.
Succos is interesting in terms of the leniencies often allowed.
I have heard from what I considered reasonably Orthodox rabbiem that as in the answer from Shalom that you should treat it like your dining room, but taken to a slightly different level. If conditions were such that it was uncomfortable to eat in your dining room, you would not. And thus it applies to the Succah.
This is very clearly a CYLOR case however!