You make very good points about the unsuitability of a marriage between Edmund and Mary. The scene between Amelia and Anhalt in Act III is full of pretty hot stuff. No wonder Edmund and Mary both wanted to practise with Fanny at first. It would have been very interesting if Edmund had practised some of the lines with Fanny.

Amelia. So you may remember I said when You began to teach me mathematics. I said I had rather not know it—But now I have learnt it gives me a great deal of pleasure—and [hesitating] perhaps, who can tell, but that I might teach something as pleasant to you, as resolving a problem is to me.
Anhalt. Woman herself is a problem.
Amelia. And I'll teach you to make her out.
Anhalt. You teach?
Amelia. Why not? none but a woman can teach the science of herself: and though I own I am very young, a young woman may be as agreeable for a tutoress as an old one.—I am sure I always learnt faster from you than from the old clergyman who taught me before you came.
Anhalt. This is nothing to the subject.
Amelia. What is the subject?
Anhalt. —— Love.
Amelia. [going up to him]. Come, then, teach it me—teach it me as you taught me geography, languages, and other important things.