Well, she didn't get him out of prison for 13-14 odd years. If she really thought he was innocent, maybe she would have tried sooner. The fact is, she was dying when this happened. I don't believe she got magically smarter right around the time she was going to die. I believe she wanted to either continue her family or save the son she loved against all rational thinking. Also, if she did believe he was guilty, I don't think she necessarily blamed him for everything. I think maybe she knew he did it, but she mostly blamed the Lestranges. However, I believe she realized he was, on some level, guilty, and his life was probably not worth her life. However, when he was dying, she might have realized she wanted to help her only child. It's selfish and loving, so that's what makes it difficult. However, I don't feel this goes against cannon Slytherin portrayals. Waiting a long period of time before the moment breaks someone into becoming emotional and showing their true feelings for a beloved person . . . I see many examples.
I just think it would be hard to convince people that she was a Slytherin.