You wasted your time with this post.
While all of what u said does make sense on a pure logical basis, the only thing you prove with this, if anything, is that the statement about nobody except oden being able to tame/use enma is a plothole cause nobody ever tried. So yeah, you could say it's a plothole by Oda but i think Oda would have an explanation for it even if he would pull it out of his ass.
After all Oda made Hitetsu say that nobody was able to tame Enma except Oden and that a normal swordsman using it would be a husk for a reason.
Kinemon was visibly frightened and warned Zoro to not use the sword because of it's power/haki drawback how would he know about that though if he or anyone else but oden never tried using it?
Arguments could be made that while Oden was in possession of Enma other Samurai and/or the scabbards felt it's power and asked Oden if they could try and hold it, which Oden let them do and immediately they felt that it was to powerful for them, that'd probably be the explanation Oda would give if he was asked in an SBS.
Also Oden was portrayed a monster even when he was a kid so therefore also an argument could be made about 10 yrs old Oden > adult scabbards.