Way, way out of my comfort zone so I’m not really gonna weigh in, but James Grier’s The Critical Editing of Music: History, Method, and Practice addresses many of these issues and questions. I have a cheap ex-library edition I got on eBay:
Some of it is on Google Books here. You might find his thoughts on the matter helpful, but I think a lot of those questions probably only you can answer. I guess I would suggest examining what exactly is the purpose of your new edition, as opposed to a reprint of an existing edition. Is it creation of a usable performance text with modern semiotic conventions? Or accurate preservation of symbols, styles, conventions, etc.? The more you can clearly describe the purpose of the new edition, the more the answers to the questions might become self-obvious to you.
Good luck! Looking forward to seeing images of the finished product!