Here's a negotiations question: Is Google overpaying or underpaying for "default"?
If you believe the price tag is so high because Google is secretly paying Apple to stay out of the market, then they'd be overpaying what would otherwise be a fair market price.
But the key argument in this case has been that the value of being "default" is so high that companies like Microsoft can't compete, since those extra queries make google way better. In that sense, Google would be UNDERPAYING.
To make your overpaying argument, Microsoft should have been arguing that being default is worthless and that Google was clearly paying Apple for some "other" nefarious reasons, like not developing their own search engine. I actually think that's a much stronger argument because I think there's evidence of it. But it's not the main argument being made in this weird case about how important it is to be default.
They could have just looked at the evidence of a deal between Google and Apple for Apple to not compete and left it at that. They've made this too complicated. The involvement of these other search companies is irrelevant.