Both facts are too weak.
1) Mastering doesn't mean he actually had the swords. Each sword has its own individual fighting style. Suigetsu might as well meant that he mastered each fighting style, but that doesn't mean he actually had the swords. Especially with Samehada there are a lot of loose ends as Kisame took the sword from Fuguki and on top of that Samehada has a will of its own. The time frame doesn't match up as when was Mangetsu then the owner of Samehada? Before Fuguki? Together with Fuguki? None of those make much sense unless we got more information.
This argument is very weak.
2) That doesn't mean he owned the swords. It might by very well a scroll, just like the swords itself, that is passed down from generation to generation or just something that serves as a precaution of some sort. It's not because he had that scroll he actually stored the swords in there all the time. The ones he summoned might be very well stored there by Kabuto himself.
Again weak argument. No proof that he ever owned the swords. Which weapon would it be then? There's no proof he owned all the swords, so it will not be allowed. But which sword did he have then? It's assumed it's Hiramekarei, but even then there are some things that don't make sense.
1) I did not support the fact that he owned every sword with the fact that he mastered them. It is a fact that he mastered all swords, and in that fact, I only need that he has the skill to wield every single one of them, I do not need to know if he was the owner of all of them, or even if he was the owner of none. In this first fact, what I need is that he is able to use them, regardless of having them or not. So if someone were to make a Mangetsu bio, he would have knowledge of all the swords, and mastery of their style.
2) Again, I don't need proof that he wielded any of the swords. If we go as you said, a Scroll passed down from generations, then he had the scroll even when he was alive, as the Leader of the seven swordsmen of the mist, and that he, at some point of his life, had all sealed in the scroll. It is true that the scroll might have been Kabuto's creation, but why would he feel the need to get a specific scroll, with all the names of the swords? Either way, if we go with the idea that scroll is passed generation in generation, if we create a Mangetsu bio, in addition to the above, we also get a character that possesses a scroll with every single sword, and the ability to summon them on his own.
Adding both together, why wouldn't a member use the swords? As far as we know, only Samehada is sentient, and thus it may not allow Mangetsu to wield it (Or it may allow because Mangetsu is the leader of the swordsmen and there could be some kind of respect bond between eachother. Not to mention that it is weird that he mastered all swords and never once handled Samehada), but all the other swords are simply extravagant in appearance and abilities, they don't seem to have a mind of their own.
