0) They do.

1) They could just adapt MPL-2.0, which provides GPLv2+ compatibility while still providing the same patent grants.

2) The upgrade is chosen by downstream users. The OpenZFS project could ask individual contributiors to choose to license their future contributions differently but that will only affect future versions and isn't a single decision made by the project leads. I don't know in what context that discussion was in but given that the have not already opted-out of future CDDL versions kind of indicates that they can imagine future CDDL versions they would choose to upgrade to.

Also, OpenZFS is under CDDL-1.1 not 1.0.

IIRC from LWN discussions, some of the newer OpenZFS code is now CDDL-1-only and could not be upgraded to CDDL-2.0 without explicit agreement from the owners of the new code.