WOW, Thinker. I have lived this SAME, EXACT situation b/w my mother and W. A lot of what you wrote I could have written.
First, you seem like a pretty bright guy, so I hope you already recognize the classic passive-agreesive nature of your mother. Avoids conflict, says she's fine, but keeps a running list of people and events who have wronged her. The building alliances within the family is part of that too - and my mother did the exact same thing. One suggestion is to view this from a different perspective - I'm not sure it is as much building alliances as controlling the family through the creation of divisive little camps who all want/need mom's approval. That is empowering to your mother (JUST LIKE MINE).
Not to be a smarta$$, but I find it ironic that your father is doing exactly what you are doing - protecting your W. As a man, he should be able to see you are doing the right thing. Maybe he can, but your mom has too much control? Would fit with the dynamic. On that topic, who is the "leader" of the family - your dad or your mother? And I don't mean the "apparent" leader, but the real one.
As I'm sure you know by now, you are going to have to stand your ground on this battle. This is a BIG one when it comes to your M. All you can do now to show your W you are choosing her over your family is to show her by your actions. Keep expecting her to vent at and on you. Until she separates you from your family, it will keep coming. And she needs to get the anger out.
In my sitch with my mother, it played out and, as usual, the truth came out about how my mother was acting and how she was pitting me and my siblings (and even my step father) against one another. While I have a great R now with my step father and sib's, that was not always the case.
