re: a 15t jump. For years I rode a bike with 50/40/24 with downtube shifters, and the 16t jump was doable, its just that its a bit too much for everyday riding. Im sure with friction shifting it shouldnt be a problem, it wasnt with the old lx fd on that bike. The time I changed my 50/39/30 to 26 posed no problem for my tiagra fd, but then this is 105 so who knows.
Personally, I think its worth doing the 36 mid ring thing, as riding loaded I figure 36 is a minimum for a 700 wheeled bike for having a more workable range of gear inches in the mid ring, where we are most of the time.
I expect the fd on the bike can be moved down easily on the frame, so if you go with a smaller crankset, it should be a fairly easy adjustment of the fd down a bit for the smaller rings.
I'm not stuck to the idea of keeping the FD, they are cheap enough if I need to change. It is a clamp on FD, so no issue moving it where it needs to go. 24 would simply be a bailout gear, not for regular use, 46-36 or so would get me all the useable range I normally need. Probably a moot point, though, if the smallest I can get a 130 in is 38, I'll probably shy towards a new crank. I can be flexible on 46 v 48, but I do agree with you on wanting a 36. I've got other uses for this part, so it won't be a waste.
Guess I probably should also ask if there is a cassette option that can get me to the range I was looking, that I am just missing. Seems my only other 9 speed touring option really is a 12-34, which doesn't change too much over the 11-32.
Never had an issue with square taper, not a concern or selling point to me either way.
As an aside, for whomever was thinking it wasn't possible to find one, there are some 46t options for 130:
https://www.niagaracycle.com/categories/sugino-46t-130mm-outer-chainring