1) Scars would remain with either option. As a shinki, he would just have no knowledge of how he received those scars in life.

2) Anything that he would innately know, he would still innately know. If it is something that he has trained in, he would remember how to do it, he just wouldn't know how or why.

3) You are allowed to make the shinki death anything you would like it to be! It does not have to be canon.