The first of my beta-readers returned P & P to me today with her notes. As I suspected, the exact same plot problem M pointed out to me, was the main problem she found. Luckily, I rewrote that part last night. I'm going to finish revising the whole end section of the book tonight to make it fit with the new scenes, then get M to go over it again. Hopefully it will work better now. I think it does, but it's easy to think that way. There is no way I can stress how important fresh eyes are.
Then, the next step is to do another pass through the whole manuscript, line by line, to make sure everything is as smooth and fluid as possible. My beta pointed out a few places where the timeline is a little unclear, and it has become apparent that the section I had at the beginning identifying the time period has been cut in one of my revisions.
So far from being finished, there is still considerable work to be done. But I feel as if I'm not the right track. At least, I hope I am!