If it can't be exclusive, that means other providers must be allowed to pay to be default on some portion of installs? If so, wouldn't that result in the basis of payment changing to a basis which takes into account the number or (e.g., advertising demographics-based) desirability of the default installs that Google receives, rather than a global amount based on what is expected to be aggregate number and desirability of all users of the product covered by the agreement?

Forbidding Google from requiring exclusivity is not the same thing as mandating that Apple accept payments from others.

Google can afford to pay more per user/click because of scale economies; their cost per user/click is lower. So, great, Google will pay Apple $20/user/year on a nonexclusive basis, and Firefox or whoever are free to match or exceed that, so long as they don't mind losing money on every user.