that sounds pretty goofy, or rather, that the wheel builder wasnt really a proper craftsman for letting low quality go in there--I mean it seems in everything in life there are people who will let underpar work go thru. I mean especially in a case like this where the small amount of dollars difference between mediocre spokes and hi quality stuff must be very little overall, but then I wonder if the build quality required for your specific usage was underpar also, or that the wheelbuilders experience was not up to snuff vis-a-vis him or her knowing what you were throwing at this wheel abuse wise (roads, weight)
I strongly suspect that someone with lots of experience building touring wheels or tandem wheels, would know exactly what to ask about the conditions the wheel was going to be used in (and especially the context, your trip I mean) and then overbuild the sucker--and/or convince/tell you that you NEED to spend $50 more on X doodad quality, or you WILL have problems....
Honestly though, did you give money constraints that was a factor? ....anyway, its done and gone, I dont know enough about wheel building to know about the specific parts you mention, or if perhaps you did some damage to the wheel with an impact or something that would have caused issues no matter what. What I do read often is that its just not the parts, but a big part is the wheelbuilders experience and technique putting a wheel together, that will make a diff in its long term life.
also, thats quite impressive about the stock wheels, quite a testament.