The reader is supposed to worry for a while that

1. Jane is only being swept away by a strong personality with Mr. Rochester, and
2. her religious beliefs are going to overwhelm her in her grief, and keep her from balance and happiness

Her relationship with St. John shows us that when tested, she’s going to stand up for herself on both fronts. Knowing that makes us feel much better about her returning to Rochester. St. John should have been the perfect thing to break her spirit and he wasn’t. Knowing that she also has money and family to fall back on now and she STILL chooses Rochester show us she’s also not just looking for security.

I agree that the pacing gets a little slow in the third act, but it’s almost all relevant. As for the religious adherence, all I can say is that it was written in another time, when those things were of real concern to most people.