ZFS protects you from disk errors. ECC protects you from memory errors. Using one or the other is safer than using neither. Using both together is even safer.
100% yes. With non-ECC you will always have bad RAM bits eventually. With ZFS this is especially bad because it can corrupt your checksums or your ZFS metadata, which means either silently corrupting your data, or corrupting ZFS itself and losing your entire zpool (akin to losing a RAID array).
Maybe not: that ZFS needs ECC is "common wisdom", but the disaster scenario appears not so likely. See