Tricky to ask if you've already gone.
And when Aunt suggested the system for paying, you could have spoken up. Anyone can with these things and if you feel strongly, you do have to be willing to, even if you find it embarrassing or don't feel you should have to. Of course if you leave early, or aren't then for the discussion, then you can't say what you think.
Someone has to say something about the bill at a group meeting. Anyone can state before the meal, how they would like it sorted. You could have done that. Often no one says anything until the bill comes. People often suggest splitting because it's easier in terms of calculations - it's not always because they want a cheap price for a big meal. Whoever makes a suggestion cannot consider everyone's individual circumstance or be expected to be aware of exactly what everyone ate when there's a big meal. Often the bill is plopped down in front of one person who hasn't given the bill any thought before. If you don't like splitting you need to speak up sooner rather than later. Many people won't mind.
Large group meals often involve splitting. Everyone knows that. If you choose to attend them, you have to accept you may pay for more than you ate and drank or you need to be willing to state sooner or later that you would prefer to split. If you have this feeling, say it at the start, not the end. This means that people might keep a beadier eye on what they are ordering and the prices, plus when the bill comes there is no shock or suddenly confusion because people haven't had a sense of the price of their meal. You didn't speak up at any point - so it's difficult to now complain.
And re drinks it is especially tricky. I know that if you've drunk tap water all night, it does t seem fair to pay for a boozer. Especially when people are sharing bottles of wine, it becomes very difficult - someone may have just had one glass of wine, but they were drinking, someone else had 3, should they pay more. You can't expect people to keep tabs like this. That's why the drinkers usually split the drink bill at least. Often a non drinker will be excluded from this part of the bill - but again you might need to speak up, because people might assume everyone was drinking and why should they notice that you in a large group wasn't. Again, helps to say up front that you are on tap water to save money and are keen not to split the drinks bill.
Will people think you're stingy if you say these things? Some might, but you'll have to decide if you value their opinion or your money more.
And re paying for a special guest, I think you need to be willing to do that if you go to a group event like this.