The answer to both the byod and permissions questions is the "tiered" device trust part from the article. You, the policy-maker decide how certain you are that a device hasn't been pwned given its provenance and user access story, and you assign a "trust tier" accordingly, which determines what resources it can access.

I don't think beyondcorp necessarily changes your incident response story, assuming you already have one.

A lot of this discussion glosses over the fact that U2F really makes this a viable system. U2F solves the MITM problem and ensures that the anyone who logs in does so with a company-issued hardware authenticator in physical communication (usually USB, but maybe also NFC or Bluetooth) with the client device. This means that even in a byod story, there's a piece of corp-issued hardware always attached. This in turn means that impersonation requires physical device theft in addition to credential theft.