I've watched a lot of videos of people shooting themselves in the head who are way less prepared than you. Most people just lift it up to the side of their head and shoot, as simple as that. Having actual MRI images and such of your head is a wonderful advantage you have over most people. Of course surviving a gunshot to the head is possible, but realistically, I think your chances of CTB with these preparations are pretty high.
The shotgun you're getting should definitely do the trick. However, as for the shaky hands part, I've seen advice on here before where people have suggested to practice shooting yourself in the head with an empty gun, just to get used to the weight, the feel, the sensation of it all, and the click of the trigger. With enough practice, the idea is that your brain and body should be able to become used to it, working on muscle memory more than a conscious awareness to execute an action.