Are there other characters besides Mary Crawford and Mrs. Norris who blame Fanny for Henry's running away with Maria? Those two are not exactly characters whom we regard as right thinking.

Fanny felt she knew Sir Thomas would think she was justified and she was right.
when Sir Thomas came back she had every proof that could be given in his then melancholy state of spirits, of his perfect approbation and increased regard;

As far as Henry loving Fanny, the narrator told us explicitly, probably because if she didn't, who would believe it?
But he went off with Maria knowing he loved Fanny, so love alone can't make him trustworthy. I agree with you, even if there had been no Maria incident, married to Fanny or not, it probably would have been only a matter of time before he descended into more scumbaggery.

Even back in ch. 35, when Edmund said but you will make him everything. , he never imagined Henry was capable of what he proved to be. Fanny said I would not engage in such a charge. Fanny knew (and warned Edmund) he was a bad guy, but I think both were shocked at just how bad he turned out to be.