hey Mr Alt (or Mrs)
I totally get the wanting lower gearing, and amongst my bikes is a touring bike that has a mtb crank like you are thinking of, and they work great for loaded touring in mountains....
BUT
you are going to have to start adding up the costs, including labour, and you might find that it adds up---now I dont know your Raleigh, but do take into consideration that if its not made for touring, you'll still have a frame that may not have the ability to take a front rack and panniers, short chainstays, and generally a frame not made/ideal for carrying more luggage if you get into that
I wonder that after looking at a realistic assessment of the final costs involved, it might be more effective to look for a good used touring bike that will have all the advantages of a touring bike designed for carrying panniers
re the actual technical aspects of your ideas. I believe most are realistic, and sora sti do work with certain fd's , but again, you're going to have to get reliable info on which fd to get and if it will go onto your frame etc
I know good quality used touring bikes are not common and they tend to get snapped up quickly, at least they do here in Montreal, and buying used can be an issue if you know nothing of assessing bike stuff to pick up on any problems....
alivio triples as far as i know, still use square taper bb's, so you could get a bb that has the right spindle length to work with the chainline issue, but fd choice may be an issue depending on how your existing frame is for the fd, clamp or braze on thing.
good luck getting proper info
and again, you are certainly going in the right direction wanting lower gearing, it makes touring so much more enjoyable.