Case conversion on Performer field messed up artists names

I fear I may have screwed up royally... My "Performer" fields were originally formatted like this:

PERFORMER: Paul McCartney (bass guitar)

I wanted to change the case of the text inside the parentheses to Mixed Case so I created this action:

Case Conversion
Field: Performer
Case Conversion: Mixed Case
Words begin from/after any of: (

I ran that action on all my files and it seemed like it did exactly what is was suppose to. Until a few minutes ago when a light bulb went of in my head and I decided to double-check my work, and as I feared my "Performer" fields now look like this:

PERFORMER: Paul Mccartney (Bass Guitar)

I've confirmed it by looking at a couple of other files and every time there's a name that involved "Mc" or "O'", the rest of the name is in lower case.

Question 1: How do I resolve this issue in the smoothest possible way?

Question 2: Why did this happen in the first time? I get that I misunderstood the "Words begin from..." setting in some way but I can't figure out how it should be formatted to achieve the desired results.

Any help you guys can offer would be extremely appreciated. All I see in front of me right now is a ton of work and no other options.

See e.g. this thread on case conversion: /t/11684/1
or this: /t/15863/1

In respect to the McCartney case: please watch out for artists like MC Sar and the Real McCoy... two kinds of MC there.

Thank you! I actually use the actions mentioned in your first link but it never occurred to me to modify it for the Performers field. That should save me a ton of time and I'm extremely grateful! I'm going to read through the second link really carefully so that I don't repeat my mistake.