Old problem, no real new solutions. You're looking at a real mess.
For some further ideas, it may just be you need some helpful keywords for your google research, you've hit all the obvious methods.
Keywords: Obsolete, objects, schema, database.
You'll find a number of blog posts and ideas there, but there's no fixall solution. It's an iterative process to get it cleaned up and you need to be able to reverse the gears at a moment's notice if an error is made.
In the end though, you're going to end up with a (server side) trace and your bosses will not be happy, and they'll have to live with that. Otherwise you just end up maintaining a bunch of obsolete structures. You've hit most of the methods already.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
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