it would be a major step for privacy, if we accept all approach as a valid, to answer c) it wouldn’t hide old transactions only new ones using the feature you mentioned.
also it takes a lot of work and careful planning.
I'm not a tech guy, nor do I have the proper skills but I'm certain that it will need time and big effort. However, that doesn't mean that it can't be done, unless some of our community gurus say otherwise.
As I said in the other thread, if something like that happened, I wonder how all the CEX's and the KYC/ AML checkers will work out and name "dirty" coins and addresses. They will find a way, or will it be a dead end for their methods?