And the last and most obvious is—
How to avoid making your parallel story plant not look contrived? By planting a few more subtle clues along the way and make the plant somewhat necessary to the story. It could have easily been insinuated that the brother was not dead but missing and somehow involved in the missing fortune/Eliza’s freedom to marry someone she could freely choose instead of the idiot she had been tied to. That’s just one idea of many, and fun to think about.