1.I would stick to flap jacks or other oaty food thats slow release since your not racing and just needing something that lasts,plenty pasta and rice pudding is great too.
2. I find Assos cream very good as is Udderly smooth but the assos feels cooler.
3. Just keep putting in the miles, use panniers in training if you're using them on the trip to get used to the weight and balance but rest the week before you go.
5. You will enjoy the trip, expect to slow a bit when you hit the highlands (watch out for the Berriedale Breas 50 miles from the end!