Off to a great start! I agree, 2.5" might be too tight. Also there aren't too many 26er tires to choose from that are good for dirt touring without getting too knobby, except for the Surly ExtraTerrestrial. The have a 26" x 46c option that might be rad. Depending on how loaded you're doing your touring you might be wary of tire rub.
Can't beat a Surly fork for the cargo mounts. Otherwise I'd suggest a Kona Project 2, sometimes you can find them for cheap, but they have no mounts.
What's your plan on shifting mechanisms? I've found that rear shifting is the easy part when it comes to mixing and matching road 10 speed/mtb 9 speed stuff. The front derailleur can get tricky - lots of variables between derailleur throw, shifter indexing, chainring spacing, and clamp diameter. I think your plan of a "secret" granny gear will work if you take an UltraRomance casual approach to touring and won't mind the extra effort of manual shifting for the sake of minimalist running gear. Or - run the derailleur with a friction shifter. Friction is the ultimate answer.
If you're going to be on the road for a significant part of your touring, I'd recommend a 11-36 cassette and a 42/24 or something chainring combo with friction for the front mech. If it's all dirt, definitely an 11-42 1x setup. In fact, if you want to try it out, I've got a lightly used 11-42 cassette and WolfTooth RoadLink we can chat about.
Keep us posted!