I think In'Youton no Jutsu involves turning Genjutsu into reality...
The Sage of Six Paths used this to literally separate his lineage into Senju and Uchiha.
- He used it to imagine the different bodies for each of the tailed beasts and then used the Jutsu to manifest them into living beings.
- My guess is Madara Genjutsu'd up what his will would look like and then used the jutsu to manifest it into a living thing... I.E. the black chakra rods...but Idk for sure...this manga is inconsistent.
Madara displayed an extensive knowledge of Yin–Yang Release, its component Yin and Yang as well as techniques which involved this kind of nature manipulation such as Izanagi; knowledge he would impart to Obito. Madara noted that the White Zetsu clones were created using the Yin–Yang Release. He also displayed the ability to materialise his will to suit his needs. Using this ability, Madara created Black Zetsu, by infusing part of his will into White Zetsu, and the chakra receivers by giving them physical form. Due to the chakra receivers being his will incarnate, Madara can control anyone who has these inside their body, causing them to sprout and cover the target's body with a black matter, akin to how Black Zetsu was first created.
In-Youton is "yin and yang" release. The combination of spiritual and physical energies. In other words, the basis for Izanagi. So you're on the right track. However I don't consider it to be identical to Banbutsu Souzou, which is "the creation of all things". It would be like saying all lightning jutsu are identical because they're all Raiton.
Izanagi would be similar to Banbutsu Souzou only in the since that you use In to create the shape of your body, and though You to breath life into this alternate version of yourself while leaving the old you behind to die. It's not the same as creating a brand new, original, sentient being like the Bijuu.
And honestly? I dunno if the black zetsu stuff can be considered Inyouton. Seems like plane old Mokuton to me. Mokuton with his will shoved inside. If it were Inyou, then he could've just created him from nothing. Just imagine a henchman in his head, give him form, and then fill him with life. Instead he was a culture grown from fleshy bits he got from Hashirama.