All those factors makes it impossible to just add a flat multiplier and be done with it.
All those factors also made it so people around here allready agreed (and stated several times) that they dont necesarily need any ballancing ... and are more than willing to "withstand" unballanced and possibly easier experience than expected.
You know, fun part is that if someone mind to have easier game ... All they need to do is simply dont turn this option on, aka ... leave it as it is ... or in even other words, do nothing.
The problem here is that people want a bigger party for reasons beyond just difficulty. Some people to be able to interact with more characters at a time, they want to see more companions' reactions to things, etc while at the same time having a satisfying difficulty experience for them. I'm not one of those people, I like my rpgs easy so I can focus on the story, butI know I'm in the minority on that matter, so it's not as much of an open and shut solution as you suggest.