I agree with your assessment of the prospects for both couples. I have posted elsewhere about a Fanny/henry marriage, so won't repeat myself. Regarding Mary and Edmund, I think they have other obstacles as well as the lifestyle-income conflict.

One thing is the moral gulf between them. Although it is suggested in ch.37 that Mary might have come to adopt the opinions of the man she loved and respected as her own, I don't think this is likely because Mary does not respect Edmund or his religious beliefs. Sooner or later he would have realised how far her ideas about morality diverged from his own, and there would have been a meltdown along the lines of their "last interview of friendship" described in ch.47. Edmund would have been just as disgusted, and being no diplomat, would probably not have handled the situation in a manner calculated to persuade Mary to think as he does.

The other obstacle I see is that Mary would have had to give up all her London friends or endure their ridicule about Edmund's occupation. We are told that after they had parted, Mary had had enough of vanity, ambition and her friends. But that was in the aftermath of their parting, so I doubt she would have felt that way if she and Edmund were married. She would soon have been bored to distraction living quietly in the country.