I forgot to mention this. If you can afford it, your normal-tools, and save-a-dive-kit tools should not be mixed. Last thing you want is to be on a dive trip, and discover you "borrowed" a wrench from your save-a-dive kit.
I'd have a dozen Knipex tools if money wasn't a factor. They're usually a little pricey.
Anything needing more torque than a 5in or 6in can provide, probably is extremely rare and the kind of maintenance you shouldn't be doing at the dive-site. I'd say either 5in or 6in should serve you well.
1. My save-a-dive kit is mostly about having tools available that will get the job done FIRST. Whether they're amazing tools is really a luxury.
2. Useful, but I'd probably leave those in my home-dive-tools. Do you really need those at a dive-site?
3. Agreed on the backup regulator. Disassemble it to save space; just a 1st stage, 2nd stage, spg, hoses (no octo). Rather than doing any field-maintnence, you can always quickly swap out the troube-part, or simply put together the regulator which takes little time. I put mine in a sealed plastic bag, to keep dirt/water/etc out of it. The main downside of having a complete regulator in my save-a-dive kit, is they do add a bit of weight.