Steel is pretty much a given, I would ride other materials, but for your criteria steel would be the best.
Carbon probably could be good also because it is repairable, without heat sources, and it is extremely durable when properly handled. The prime example of the latter is the way carbon has come on as the toughness winner in archery. Arrows hit things at 300+ fps, and they don't always survive, but carbon has risen from the most fragile material the the toughest of the lot. The problem for bikes is that few if any users of carbon are doing the work with durability as the main issue. Carbon tends to fail spectacularly when clever prototypes hit the real world, so it would only be great once the first few thousand frames had been around the world and back, and even then the real advantage over steel might end up being slight. "better" materials aren't always the best choice in every case. Carbon is the obvious choice for golf club shafts, but it has proven very tough to implement. Here is an interesting example of carbon repairability:
http://www.youtube.com/watch?v=mx0Rw_RqXCo
Start about 3:30
I don't see any particular reason to favour lugs and butted tubes. Welding is totally proven in this kind of application. So are lugs and brazed joints. Just isn't a major issue as far as I can see.
Butted tubes can make a lot of sense but if you run the numbers, the weight saving is pretty minimal so even butted tubes may not be necesarry. A lot of the fancy tubes in tube sets, other than the main triangle are not really butted. They have wall thickness differences but not in the same way as in the main tubes. A lot of it has to do with getting tapers into the tubes for various reasons, but it isn't the heat wall thickness issue you see with the main tubes from what I can see. If one could find appropriate straight wall tubes it has some advantages for all the braze ons, and stuff like S&S. Maybe advantages is going too far, but it might do no harm. I'm looking at it from a large rider perspetive. Start to talk about 1.1-.8-1.1 tubes, or whatever, and good old .9 straight wall starts to look pretty good.
One thing about round the world trips is that they are high mileage low time duration events. 3 years, say, is not a lot of time if one is worried about stuff like rust, one of the few ways in which 4130 type tubes are not supreme.
For a world tour I would put a lot of effort into the design, the fit, and not too much into the material, 4130 type steels would suit me fine.