They say that the road to hell is paved with good intentions....
Both Taronyu and Wiliam are playing it safe, it's just that their definitions of "safe" differ

I can see merit with each version, but I can also see that there is a possibility for unity....
Taronyu asserts that you must not use -ru lu for possession. That is safe because there is nothing in known existence that supports the validity of -ru lu for possessions. Going by this you won't accidentally use it incorrectly.
William asserts that there is nothing in known existence that negates the validity of -ru lu for possessions, therefore why should it be forbidden ? Going by this you might accidentally use it incorrectly.
From an "absolute safety" point of view, Taronyu's assertion is "safer" than William's; it is better to use a sub-optimal correct expression than to use an incorrect one. That said, I don't think that William was actually advocating the use of -ru lu with possessions, but merely suggesting that forbidding it, without proper evidence, would be incorrect. It is also entirely possible that -ru lu
is actually valid. We just don't know.
Perhaps the answer to this is to turn the rule into a recommendation ? Ie "you can use -ru lu for states, but it is not recommended for use with possessions due to lack of attested examples". This is "safe" on both counts, it tells you what is definitely known to be correct, and warns you of things that might be wrong without actually forbidding them. Food for thought?
Eywa ngahu.