I might redirect you to other comments that touched on some of your topics

This comment from u/hborrgg touches on the views contemporaries had on the various Japanese weaponry and some of the Ming and Joseon units https://www.reddit.com/r/AskHistorians/comments/5mjnfe/comment/dcbdsmn

As for why the Ming didnt adopt arquebuses is just because they had their own kind of muskets, what divided the Japanese and Ming was their philosophy regarding such weapons. In pre-modern ages, it is natural in the management of a such a massive political entity as the Ming that things are bound to be not uniform. Tha Ming had millions in terms of manpower, and those were recruited from different sources in different contexts.(according to their regional laws, and such). As a result only some would be armed with arquebuses, if they had, while others would only have extremely primitive firearms.

Also the Ming muskets were notorious for their unsatisfactory manufacture quality. In Qi Ji Guang’s manual, it was said that soldiers were afraid of aiming, because the muskets may easily blow up. While those ideas were overblown, this, alongside the previous point made creating entire cohesive units of trained musketeers extremely hard and inconvenient when they had already the upper hand in manpower and most importantly, artillery.