Why does Photos keep using OLD library?
It looks like you did everything you should.

I have many times made a new photos library, set it as the system library and then rebooted so you're on the proper path there. My hunch is you have iCloud turned on for that library as well and need to resolve that.

I would turn off photo stream and shared photos and restart one time to see if that resolves. If not - you'll need to disable iCloud photo library and walk away from the broken local library or try rebuilding the affected library.

Worst case, contacting Apple support sometimes helps. They often know of edge cases like this where things get stuck and can advise you some hidden tricks to get the libraries unstuck.