The Horde Rebellion IS an internal matter. The war against Garrosh in a broader sense is not though. I agree that after multiple Ashenvale invasions (Wolfheart and Cata quests), invading Gilneas, bombing Theramore etc, going after Garrosh is just as much if not more a part of their story than the Horde, who are only just now being pushed to full on rebellion. So you're right, cutting the Alliance out of Garrosh's end and making it a Horde only story would be terrible.
Not sure I agree about making an Alliance camp in Durotar though. Don't get me wrong. I would LOVE for blizz to add in a couple more quests, with us sabotaging the Kor'kron war effort against the Darkspear, not only bringing them supplies but also weakening them directly, giving the alliance more content. But I don't think that the Alliance setting up a camp, even small, in Durotar would make much sense. Really the only problem I have with 5.3 is the alliance needs a couple more quests working against the Kor'kron.
I will disagree with one part of the OP though. Varian is very interested in what's going on in Durotar. He even sends a letter to you after you finish there, following the si 7 reports closely. But he's a king, not a spy, not an si 7 agent, why the heck would the high king of the alliance be on a small scouting mission instead of, for example, securing more dwarven troops for his war against Garrosh? That's like asking why Garrosh isn't picking his new toy up in person in the Vale, guy's got other things to be doing right now.