Don't know if this delivery guarantee is really required in all use cases.
Yes, exaclty my thinking. It also introduces a lot more complexity.
If one argues that "most available" is the resource (= aka cluster node) that had the least work in the past
Good point, will add that argumentation to the XEP.
Concerning your question. The hash-based approach might be implemented as follows.
I understand the approach. Hashing is a costly operation, and now you need it to do for every message stanza. What I don't understand is where you see the advantage compared to simply doing round robin?