I can't help with your first two points. I barely notice the cobbled sections myself.
On the other two;
3. For the most part it's a faster pack than London that's true, but there will be plenty of people at the pace you ran London on the course and there will be people employing a walk/run strategy.
4. The finish area is carnage in terms of meeting someone. The supporter group from here tend to travel the course to see people at a couple of points on the course and set up camp at the park just before the finish. You could arrange for your family to meet up with you there, or pick one of the streets off the side of Ave Foch as Foch will be full of runners, people trying to find runners and food stalls.
2. Experiment. Use your long slow runs to see what works for you. Try some different brands of gels, shotblocks, jelly babies or mix and match. In terms of what your stomach can handle on the run tends to be very much down to the individual.
3. Foot pod would be most accurate. Personally I'd only use your watch for time, not distance. Use the race course markers for the distance. You can pretty much guarantee tunnels or otherwise your Garmin will measure longer than 26.2 on race day unless you've been glued to the blue line.
4. I'd say a 1:57 half would put you in the 4:10\4:15 range. Best method would be to see how your training is going at certain points and you can revise your target time up or down depending on how well things are coming along.
Thanks LLoyd R. My Mac isn't compatible with garmin express. Le sigh. I'm in the process of trying to upgrade it but I fear that this is the end of the road and it is no longer supported. However, it is compatible with my phone, and when I look in my settings it says I only have one device - my new one. Honestly, they don't make it easy, do they? I can't even see the settings to turn off manual upload for my 410, so I think we'll have to upload Mr KS' using the nudgey manual button and then delete his runs from my records. That is, until garmin phase out ant+ then we're really stuck.
What Mac are you running? There's a Mac version of Express to download on the Garmin site. http://software.garmin.com/en-GB/express-download.html#mac