...and what if not everyone agrees?
There has to be an agreement in the end. If you're not amenable to that, perhaps best eat at places where you pay in advance for your food individually, there are probably more of those than where you pay by the table these days.
Surely the only sensible default is 'pay for what you had, and add a reasonable tip'. As someone who is teetotal, for example, I'm not going to pick up a share of the bill for everyone else's three rounds of beers or wine. Equally those who don't have dessert shouldn't be paying for those that do. Those who have a sandwich shouldn't be paying for a share of someone else's steak. Etc. etc.
In your opinion. Most of the time when I eat out it is just agreed to split the bill, though if anyone isn't drinking alcohol often that's separated out or we work out a discount for the non-drinkers. Half the time soft drinks cost as much as alcohol anyway, particularly where Coke etc are served in the small bottles so a soft-drinks drinker typically gets through more than the alcohol drinkers do.
I usually find, though, that if one person does insist on paying for their own then that's what is agreed by default and a menu is requested to enable everyone to do so. But usually people want an easier option to simply split, I find.
Tipping I've always found to be agreed as done individually either when the card machine gets to you or by leaving cash, as individuals prefer, given that there is no consensus on tipping in the UK. The one time this really wound me up was when I stated I wasn't tipping due to poor service (I usually do, but not if the service is bad) and someone else loudly said "I'll put in for you then" in an attempt to guilt-trip me and did so, I gave him a proper earful about that and made it clear that under no circumstances would I be reimbursing him for such a thing.