How to measure and treat BSA bottom bracket housing misalignment?
The threads on bottom bracket shells can have angular misalignment. The threads are typically cut in two operations, one from each side. If the shell is not aligned properly in the jig when the threads are cut misalignment is the result.

Misalignment should be fairly easy to diagnose at least.

Install the cartridge only from the drive side making sure to not cross thread it. If you look at the non drive side you should be able to see if the body of the cartridge is centered in the shell on that side. That will tell you if the drive side thread is aligned.

You can also install the lockring and see if its flange lines up with the edge of the shell (this is not a great indicator of thread misalignment , because the edge of the shell may not be true).