Christian, do you want that I share my opinion, than please tell me your decision. I`m not talking about „high speculative“ interpretations. The blade is like other Naotane soshu blades. The NBTHK said „mumei Naotane“ not den-Naotane, and they had some specialists doing Kantei on a lot of Naotane blades ( think about 30 Juyos ).
Again for that money, esp. when you start collecting him, its a seldom possibility. You will never find a signed and dated wakizashi under 1,5 million yen in Japan. Shipping and customs and fees and you have to pay nearly 2 million yen only for a good TH sword. You can ask Ms. Shiho about the blade, she is a friendly seller and can give advise..
But the blade is sold, the second mumei Naotane. The first one I saw was a mumei katana here on NMB. Have some TH Naotane and we learnt to collect the Japanese way, where in it is said.. only buy better swords.
Best