What are the advantages of a custom frame for a touring bike? I'm building a new bike for touring while traveling, so I want a break-apart frame with S&amp;S couplers, like the Salsa Vaya Travel, or another steel touring frame retro fit with couplers.  I want a bike I can confidently take down the long roads knowing that if anything does go wrong, I'll be able to fix it on the road and get rolling again.  I also want a bike that I'll be able to ride for a lot of years, without investing so much in it that taking it out on the road will seem like an unreasonable financial risk.

So, my question is: should I spend the extra money and time on a custom built frame, or will my results be just as good with a well fit stock frame and components?  What advantages can a custom fit provide that are unavailable in stock frames?  Over many consecutive long days in the saddle, do the benefits of a custom fit pay off any more or less?  Are there any risks or complications involved with building and maintaining custom frames that I should be concerned about?
Unless you are of extremely uncommon proportions, you won't find much riding advantage in a custom-made bike— there's plenty of room to tweak a mass-production bike for the a perfect fit through component swaps. Moreover, unless you buy from an experienced touring-bike specialist, it's entirely possible that a custom-designed bike might have little annoyances that'd get caught in the prototype phase for production bike.

Most great cycling adventures have been taken on a mass-production bike of some sort. I rode across the USA on a used $300 Fuji, and it was fine (though a better wheelset would have been worthwhile had I had the budget). I've taken medium-distance tours on a 30 year old Schwinn Varsity, too, which meets all your criteria except packability— that bike fits fat tires, has comfy relaxed geometry, and you can fix pretty much everything on it with an adjustable wrench. By all means, if you have plenty of money and would enjoy a custom bike, buy one, but if funds are limited, I'd spend them on travel as opposed to the bike.