Thanks for this DIY. I am following it in approximation for an M3.
Just wanted to share my experience with trying to take the pump apart. I tried the pry method with a thin screwdriver, no luck. After about an hour of trying I smashed the **** out of it with a hammer. My logic being if I couldn't get it open to repair it I might as well see what it looked like inside.
The motor turned out to be completely rusted out and bound up; but after a bit of jimmying I got the armature free. So I could have fixed it.
You will need to vice clamps that you can twist separately with a lot of forced to properly get this thing apart if it is seized. Prying will probably not work like it did for the OP.