Your crank will accept a 28t or 26t inner ring, and various posters on these forums have installed a smaller inner ring on their road triple cranks when they've wanted the lowest possible gearing. But you really shouldn't expect the same quality of shifting. Shimano designed the stock ring combination to work together as a system, and any modification from the stock arrangement will have an effect. I'd expect the upshift from inner to middle ring to require slightly more patience due to the larger tooth difference, especially with STI shifting. I'd also anticipate the increased possibility of dropping the chain off the inner ring. At minimum, I'd make every effort to get the FD adjustment near perfect, and I'd install a seat tube-mounted chain catcher (e.g., Dog Fang, Third Eye, etc).

As fietsbob suggested, bar end shifters with a "trekking" crank (Sugino XD2) would have provided highly reliable, fuss-free shifting with practical gearing. But I understand your issues of time and money, hence the thoughts of switching out the inner ring.

Try searching the forum for firsthand experience with 52/42/26 and indexed shifting. Or maybe ask in the touring subforum.

Oh, you mentioned possibly switching the middle ring, too. To my understanding, Shimano made their 42t middle rings to work with one type of 52t large ring, while a slightly different design of 52t ring was used in combination with 39t middle rings; thus changing one would theoretically mean changing the other when used with indexed shifting.