I have about a dozen trips of a thousand miles or longer over the years, all on a Suzuki V-Strom 650, averaging 550 miles per day on trips, occasionally much longer and more often a bit shorter. The most important thing is to minimize fatigue, which means ergonomics, hydration, very regular breaks (every 60-90 minutes), stretching, and noise management. I keep a water bottle and a CamelBak for hydration, and use earplugs and a good windscreen or fairing for noise management. A well-fitted helmet also keeps the noise down.
1. Aside from during a long road stretch, soap and warm tap water on a visor are fine. When on the road, you can take a lint-free cloth, and soak it in some appropriate fluid (even water) and keep it in a sealed plastic bag to take out and use.
2. RainX works on some visors but might damage the coating or visual quality on others. I don't know specifically, but most motorcycle-related stores have some reasonably good cleaners on hand.
3. You can invest in bar risers so that less of your weight is loaded through your arms, shoulders, and neck. This is one reason I have a V-Strom and Zero DS. I would not expect a Zero S to be ergonomic, and you should focus on doing regular stretching before and after a ride to compensate for it.