To me, the tense pairings that make sense are:
If the AC let the buses stop... mountain, the tourists might have difficulty... (not an actual situation - what may happen if they do this)
The AC let the buses stop... mountain, so the tourists had difficulty... (actual past)
Because the AC lets the buses stop... mountain, the tourists have difficulty (or are having difficulty) ... current actual situation
By the way, two other points:
1) "Let" is an interesting choice. That generally means give permission for something you are looking for. My mother let me stay out until midnight. My boss let me have a day off. You may want to use "permitted."
2) Your phrasing "They let... so that" makes is sound like the difficulty was the desired outcome. I'll let you borrow my car so that you have a way to get to your sister's.