Many protour stringers are not certified because as @cluckcluck said, it really doesn't matter at that level. The certification is just a piece of paper. As for the ProTour, from my understanding it is stringing 3-4 rackets within a given time frame and having the DTs all within 1. I would think there would be more to it, but I don't know for sure. I've been told there are only like 5 testers for that cert at current and none near me. I haven't done the Wilson training but I spend a lot of time communicating with a friend that is on the Wilson team so I don't know any true specifics other than there is a Wilson way and it's something you would never pick up on, on your own. They basically reteach you to string within their format. I haven't had the opportunity to go through it yet but there's always time.No worries, I would be just as excited! I'll answer your questions as best as possible.
This was a mixture of proving myself on social media as well as having some good connections to be introduced to the right people; sprinkle in a little pestering and boom got the invite.
Personally, I have the MRT certification but I don't think all the stringers have one. I believe being a good stringer is the most important thing. You have can all the certs in the industry and still be a crap stringer. Proof is in the pudding, ya know? I know that the URSRA has recently added the Pro Tour Stringer element to their tests (ERSA has this already). Though I think that is more speed and tone more than anything else. Don't quote me on that part as I have not researched it enough, perhaps @SavvyStringer can comment.
There's really nothing to prepare you for a tryout like this. Yes, you know that you'll be stringing a lot of racquets but the pressure and timing of everything is something that you can't really prepare for. It's intense and can break you emotionally (there were moments where I felt like I was just going to break down in tears and I'm pretty emotionally level-headed).
I believe I will be changing my practices, it's a simpler method that if done right, I can shave off a minute to a minute and a half off of my stringing time (currently 18 min average for 18x20), just have to practice more. I have already strung several racquets since I got back with the method taught, so I may keep at it for a while and see how I like it after a few months.