Excellent points. According to historical references there was a signed Masamune blade, dated 1314, so he was active for several decades.
The condition of this blade will almost certainly hold it back. Very speculative pricing for a blade that, taken on its own merits, is far from appealing.
This is the best 'Masamune' that I've seen on the open market. Very healthy and I think this is worthy of its attribution at Juyo level, also with early Honami family papers. The distribution of Nie is quite distinctive.
I doubt there would be Sadamune without Masamune's influence. Their style from what I can tell is quite distinctive compared to the other contemporary Soshu grandmasters. Who else had such control of Nie distribution? Of course with so much cross pollination of ideas and technology a great deal of overlap should be expected. Norishige with his Mitsukawa hada is probably the standout Soshu smith offering a unique style later in his career.