Ambiguity around rotating keys and deleting metadata

In section 5.1.9, it states:

1.9. If the timestamp and / or snapshot keys have been rotated, then delete the trusted timestamp and snapshot metadata files. This is done in order to recover from fast-forward attacks after the repository has been compromised and recovered. A fast-forward attack happens when attackers arbitrarily increase the version numbers of: (1) the timestamp metadata, (2) the snapshot metadata, and / or (3) the targets, or a delegated targets, metadata file in the snapshot metadata. Please see the Mercury paper for more details.

There some ambiguity here:

• We should define what it means to rotate a key. Do we only remove the metadata if we add a new key and remove an old key at the same time in a root metadata? Or should we also delete the local metadata if we remove a key?
• In the case where we have multiple keys and a threshold > 1 for a role, do we still delete the metadata if we only rotate one key? Theoretically, it should be safe to rotate less than the threshold number of keys in a given root metadata, since an attacker shouldn't be able to perform a fast-forward attack with less than the threshold number of compromised keys.
• Why do we delete both the local timestamp and snapshot metadata if just the timestamp key, rather than just deleting the timestamp metadata? Similarly for just a snapshot key rotation. Does this protect against a known attack, or is this more about cleaning up attacker controlled files that might contain other unknown attacks?
• Why do we not delete the targets metadata if those keys are rotated? (I think this might be addressed though by @lukpueh in #65 or https://github.com/theupdateframework/specification/commit/84103fcc726eece4fae3f7876a493bc1e988db3b if that gets merged in).

Thanks again!