Fingers crossed right? That’s what all(some of) the delays were for. Test test test. You can’t test for everything but you can get close and the rest is hope it works. You can be sure they have a full size test article that is exactly the same that has tested every procedure.
I’m with you though, it never ceases to amaze me that despite all of the testing that any of our missions ever work. Hitting Pluto is a perfect example of holy shit we kind of know what we’re doing. Or the fact that on Cassini there was an innate gimbal error on it’s engines and orbit determination could detect that their actual velocity vector and their predicted velocity vector were off by only a degree. It’s the stupid little stuff like that that constantly amazes me that we can detect and account for. I wonder what unforseen issues JW will run into.