Welcome, Ian, and congratulations and encouragement in advance on what will surely be a wonderful trip!
As for suggestions, my first one is to quickly grab a copy of Stephen Lord's Adventure Cycle-Touring Handbook, 2nd Ed., available from Amazon and a number of online vendors. It has route suggestions for the area you are going, and covers many of your questions with entire sections devoted to equipment and preparation, route planning by region, and accounts by seasoned travelers. He even addresses the Visa issue to some extent, by region.
I would also suggest taking a look at each section of this Forum, paying particular attention to those devoted to panniers and luggage, and lighting and battery charging from a dynamo. Chances are, a lot of your questions will have already been answered -- or what's already there will stimulate new questions for you.
Beyond that, we're a friendly and helpful lot, so feel free to ask as questions occur (more will).
As for helmet use...I always do, some always don't, and some sometimes do. It is a matter for personal preference, one's level of risk-aversion, and past experience. It is a proven source of fuel for flame-wars, but it boils down to what you're personally comfortable doing. Very few people will ever change their positions, no matter how heated the exchange. You can take a poll and decide based on the number of "yeas or nays" or just decide to take one. That is what I usually suggest, since helmets can be hard to come by in remote areas. If a person finds it valuable to do so, they'll find themselves wearing one. If they don't, they can send it home or give it away.
On combating boredom...I enjoy any number of activities while on-tour and in camp, including bird- and wildlife-watching, photography, maintaining the bike, and planning the next and subsequent day's routes, and journaling my day's adventures. I've found that's more than enough to fill up my off-bike time, but I do carry a book and limit myself to a chapter or so a night so I have something to think about on the next day's ride. The farther you go from your present home and experience, the more basic life will get and the more you'll find to entertain you along the way. At least, it has worked out that way for me and those of my acquaintance. The deeper you go into unfamiliar territory, the greater the opportunities for self-education through interaction and talking with those you meet along the way.
If I had one tip I can offer, it is this: Be open to new experiences and be aware there's many ways to do things; while folks in other cultures might do them differently than you, their ways probably work fine for them. View this as an opportunity to grow each and every day; you will!
A second suggestion for the price of one: It's great to plan ahead, but not everything can be planned-for, so be flexible and adaptive.
A third: The best tool in your kit is a good attitude. Remember this quote from GK Chesterton:
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered."
All the best, and once again -- welcome and best of luck.
Dan.
