We've never carried in sandwiches or meals, but when our kids were younger I carried in lots of snack items. WDW "official" policy may say no, but they really do not seem to care one way or the other. They
WILL stop you from carrying anything in in glass containers. And I think they will stop you with alcoholic beverages. But otherwise, you would be fine.
We used to carry in frozen box drinks - they would thaw and be perfect by the time we wanted them. And peanut butter and crackers and other such snacks. I've never carried in a cooler. To me it wouldn't be worth the hassle, but I have known others who have done this. If the money is an issue, then do what you must do. No one should look down on you and WDW won't mind as long as you don't bring in glass or alcohol.
We still carry in bottle water since the FL water upsets my daughters stomach (the rest of us are fine). But the bottle is plastic, not glass.
As far as using the picnic tables at out door restuarants: if it isn't crowded, use one. If it is crowded, then those should be used first by the folks buying the food there. This is an easy fix - simply eat at an "off" time...around 11am or after 1pm. Or sit on a bench to eat.
Enjoy, take your food if you need to and don't worry about it.
