The whole vacuum system is linked, yes. I have a friend who carries a collection of golf tees to plug leaks in his 300SD to keep the vacuum going to essential bits (like the fuel shut off and brake assist). They're not on the same lines, per say, but they're not isolated circuits.

I couldn't tell - did you rule out GPs? One trick you can do is to cycle them several times in a row prior to attempting to start (2-3 times is sufficient). That will usually get the car started, even if you're 2 plugs down. These cars seem to do OK with one plug down, but if you get 2 or 3 down, even in mild weather, a cold start can be a PITA.

One the fuel system, you can do a quick check by pulling the canister-type fuel filter in the engine bay. There should be fluid in it. If there isn't, you have a problem in the fuel line somewhere. You can test the line from the tank to the engine bay via the primer pump - it's the knurled knob next to the injection pump. You have to unscrew it / twist it loose, pump until you can hear the bypass valve opening (it sounds kind of like a bike pump), then screw it back tight.

Get a cheap LED flashlight from Kragen/Autozone/Napa/[parts store of your choice], they're pretty essential on the road. For ~$5 you can get one that's on the end of a magnetic pickup wand dealie, and I've found those are incredibly awesome to have around. Makes finding nuts (like the little ones on the GPs that like to fall off when you're putting new plugs in, which then hide under the IP) much easier to find.