Hi, G.. Well, there are simple parts and complex parts to your questions. First of all, it's a very, very bad idea to take to heart anything that an angry person is spouting off about. Your mother-in-law may or may not have said those things, but what does it matter? Even if she is complaining about you when you're not around, what does it matter? She's entitled to her opinion, even if that opinion is negative.
It seems that it is no secret that the ex-girlfriend carried tales to you about your mother-in-law, and mom-in-law is trying to make peace with YOU. If I were in your place, I would make peace with my husband's mother because she is a relative, and this ex-girlfriend of your brother-in-law is not a relative and will probably not matter to you at all or be involved in your life next week or next month. So consider who is most important to you, and don't worry about opinions. Sometimes ppl let off steam and say things they don't mean and aren't true. Consider who is actually important to you -- that's your family.
Now, if your husband is having a problem with a growing addiction to pain killers, that's another issue. He needs to go to a doctor and discuss pain management. Understand also, G., that ppl who do not suffer chronic pain do not understand what ppl who ARE sufferring chronic, long-term, bad, unhealable pain, are going through. He needs your understanding and support to do the right thing for himself and his family. He needs to find ways to get healed or to manage the pain without getting addicted to pills.
I hope the family can forgive and rally around one another, and that your husband gets better.