There are instances where completely replicating an environment in an air gapped lab is acceptable. For instance, you could be encountering some edge case where a bit is being flipped in an IP address, or a bit is stuck on somewhere and replicating the environment down to the individual IP addresses plays a huge part in reproducing the issue.

An edge router is not one of these cases though.

.local is perfectly fine according to the IETF https://tools.ietf.org/html/rfc6762

For mDNS, yes. There are resolvers that don't even try DNS when looking up a name in .local.

Some of them were made more lenient because just as Microsoft finally stopped encouraging this nonsense, the Kubernetes people picked it up from ancient Windows Server folklore and apparently decided to make it web-scale.

As you linked, it is reserved for mDNS.