This is so weird. Who doesn't serve vegetables of some sort. Odd.
I have quite a lot to add.
My in-laws would bring their own food and not eat what we prepared, when we were first married. That's because my FIL is a controlling jerk and had to make a point and he liked 'his' salad the way his controlled wife made it (veggies cut just so), so even though we served tossed salad, it wasn't tossed HIS way. And so on, down to the bread.
You know what? Fine. Less cooking for us.
In my family? You bring what people ask (by the host) and then you often bring more than that, just because - it's fun to show up with something unexpected - but that won't throw the host/hostess off. Wine, salad, squares, another appetizer, whatever ... more food the merrier.
If it were me, I'd pack food for trip up and eat away. I would probably (since they sound really odd), just eat what was served. I would do this because it sounds like drama otherwise. I would just eat the ham and take enough of the rest to play around with it, then throw my napkin on it and be done with it.
Then I'd eat from the cooler I'd packed on the trip home.
That sounds like a very long day.
Hang in there. Remember to pack some goodies in that cooler to get you through. Be the bigger person! You can do it.