I don't believe that we know the history of the rift between Don Eladio and the Salamancas. My guess is that Gus' plan originally when he took over all transport from Mexico was to try to cut the Salamancas out by getting Hector angry enough to defy the Don. Nacho messed with that, so he did an audible, and now his plan is to cut off that transport and make it look like it wasn't his fault (which he would have done eventually, as he already looked at the laundry). Then he can just casually mention that he has people right here in the US who can make it here and save the transportation issues - what luck, and an even bigger part of the cut for him. I'm going to guess that Nacho changing alliances will not help with the rift between Hector and Gus, and I think that Gus is going to sour Hector's relationship with the Don slowly and using Hector's anger and pride against him (esp since now he can't talk). Short term, I think Gus will have whoever is making the crappy meth step in as the US supplier, and then will eventually get Gale involved.
I think Kim's reaction to the models was concern at the size of the project for her. I think she went to the courthouse either to try and slow their roll somehow, or to talk to someone into coming into practice w/her. A potential outcome of this may be that enrages Jimmy that she's bringing in a partner, and drives a wedge between them.
I think the letter was from Chuck in the past, and I think that Kim was weeping for who both Chuck and Jimmy were back then, and how far they and their relationship fell, and the tragic outcome for Chuck.