I can't suggest anything for your nan... thats a situation i know fairly well, and have never managed to defeat. Be sure to chat about it with anyone close to you who could understand beforehand. Be as relaxed as possible before it happens, so that it takes longer for you to lose your calm.
The second thing sounds better... the fact there are so many people is in a sense an advantage.. your more able to move away from people, you don't really have to stick anywhere long, there may be a bar tab. Find a senile and alcoholic relative by the bar and stick with them. That would be ideal if you have one on hand. If not, i'm sure you can find someone to stick to, whether they be a black sheep of the family who doesn't get on with your dad too much, someone your age who doesn't want to be there either, etcetera.
At least it'll be over by sunday.