I think encrypted.google.com doesn't use AMP yet, so they could make that the default search

That requires releasing new versions of iOS. Google can work around that way faster than Apple can try to play catch-up. It's a losing battle.

It doesn't require it, except that Apple has failed to decouple Safari from iOS, and so it can't be updated like regular apps.

Apple has certain things that they can push any time, such as the lists that safari looks at to find out if websites are dangerous or some of the security stuff they have.

However they ship it, they'd still be in an arms race against google to keep AMP pages hidden from Safari users. That doesn't seem like a productive use of anyone's time.