23 NM (17 ft-lb).
And this is an Amazon link to the fuel socket kit I used.
You only need the 17 mm socket, but I'm a sucker for adding tools to the tool box.
FYI - here is a link to a forum posting that had a lot of good information on the task. It's for a 335D, but the engine (M57) is very similar. Some differences in the engine bay, but no difference with the fuel injectors themselves. Posts 22, 38, and 39 will sound famiiar to you. If nothing else, it's nice to know you're not alone!
The fuel line socket is a big help - too much torque and you may cut off fuel. Too little torque and you have a leak and can't sustain pressure. As noted in post 39 - a quarter turn can make the difference.
No idea about how to check the hpfp. However, I have seen so many discussion points about the difficulty of cranking after injector replacement, with eventual success after lengthy bleeding efforts, that I am still >90% confident that is your problem. Trying to nail down how to do that properly delayed my working on the valve cover replacement for at least 6 months. Even being really anal about bleeding and torquing I was still very nervous when it came time to start the engine. In my case, fortunately, it wasn't a big deal.
Good luck.