Martin, are getting worried?
1. Not a good idea as you've never tried it before. If you haven't had any problems in this area then you shouldn't have to worry about it anyway. Just go before you start.
2. Depends how they intend handing out the water. If it's those silly little plastic cups you may not get enough water. Take a cut down straw with you, just in case, as this allows you to get a full cup at each drink station. + gels, unless they are handing out full strength energy drinks at the water station.
3. For your first marathon it may be useful having your HRM to ensure you don't do the first section at too fast a pace. You could then ignore it after maybe 15 miles or so. It is soooooooo important to start off at a realistic pace, no matter how good you feel.
Good luck and can't wait for the report.
4. Whatever shoes you feel most comfortable in.