I'm so sorry you are going through this. I think it is very painful when the A's family turns on you. They should be supporting you and wanting to help their son. Unfortunately you becoming the bad guy in the situation is far more normal - from what I have seen on these threads and my own personal experience.
If they believed you - then they have to step out of their denial and face the facts that his drinking is a big enough problem that it will destroy his marriage. Way easier to get behind that you were just using him and are now trying to get out of it by claiming he has a drinking problem. It's a lonely place to be.
I agree with what was posted above - who cares what they say - you know the truth. Do not let their denial make you question what you have been living with and going through. I have never been through a divorce, so I may be naive about how it goes, to me it seems like this wouldn't even be an issue. The judge will see through the smear campaign. Paying your student loans was a decision you guys made as a couple that makes sense for the future. End of story.