"For the sake of keeping this post as short as possible I won't go into the zanpakutou arc involving "Muramasa" but I have completely reasoned out all the "potential" plot holes and the arc is fully capable of being explained by kubo if he feels he needs to"
Tite Kubo did not author that arc, it is a filler arc for the animie only and has no connection with the story that Kubo creates.
Everything of course is up for speculation but some things are in front of us.
First we can draw a connection between chapter 541s Old-Man and chapter 411s young "Tensa Zangetus", or Young-Man(?). While Ichgio first visited his innerworld in Bankai form, he faced off against Young-Man. Young-Man stated to Ichigo that what he wanted to protect was Ichigo himself but was never explained. Old-Man in 541 explained to Ichigo why he wanted to protect him. Thus it's easy to conclude that Old-Man and Young-Man are the same.
Things that cause much speculation right now with Ichgio is:
1: Why did Old/Young-Man fuse with Hichigo?
2: Since we now know that Ichgio is being revealed his true Zanpakto, does this mean he never learned Bankia?
In reference to point number two: Shirosaki has been revealed as the source of Ichigos true Shiginami powers. Shirosaki, aka Hichgio, has never revealed his name yet also stated that he IS Zangetsu. However, it was Hichgio that taught Ichgio about Getsuga Tensho, the same Shiginami ability Ichigo's father uses. And, everytime Ichgio went Bankai prior to vizard training, Hichgio became closer and closer to Ichigo.
It's safe to say as of right now that Ichgio's Shiginami powers derive from Hichigo: A collection of Shiginami souls fused together and holowfied by Aizen, which have been absorbed by the mother and passed down to the son as a Shiginami entity thanks to the fathers constraint.
I would also like to point out that Final Getsuga Tensho is a Shiginami technique passed by the father to Ichgio. (Speculation) It makes sense the reason Old/Young-Man had to fuse with Ichgio (The true Shiginami powers) to perform FGT. After all, Old/Young-Man would need Hichigo to perform Hichigo's technique.