That is a really good analogy about carrying the cross. I believe you are right. In chapter 27, after she finds out that only Edmund's necklace fits and then decides to wear Mary's (Henry's) also:

"His, therefore, must be worn; and having, with delightful feelings, joined the chain and the cross—those memorials of the two most beloved of her heart, those dearest tokens so formed for each other by everything real and imaginary—and put them round her neck, and seen and felt how full of William and Edmund they were, she was able, without an effort, to resolve on wearing Miss Crawford’s necklace too. She acknowledged it to be right. Miss Crawford had a claim; and when it was no longer to encroach on, to interfere with the stronger claims, the truer kindness of another, she could do her justice even with pleasure to herself. The necklace really looked very well; and Fanny left her room at last, comfortably satisfied with herself and all about her."

She is now sheilded from any evil coming from Mary or Henry and can wear the other necklace too and she is finally "comfortably satisfied with herself and all about her".

When I think about when Mary gave Fanny the necklace it is really a very uncomfortable and almost painful scene. Mary wanted Fanny to pick one of several of her necklaces out. Fanny tries to take the one she thinks Mary would be able to part with the easiest and thinks Mary has been showing her one necklace in particular more often than the others. So Fanny decides upon this necklace, which turns out to be the one Henry gave Mary (which Fanny then doesn't want). And then the way Mary tries to con Fanny into keeping the necklace:

“Well, then,” replied Miss Crawford more seriously, but without at all believing her, “to convince me that you suspect no trick, and are as unsuspicious of compliment as I have always found you, take the necklace and say no more about it. Its being a gift of my brother’s need not make the smallest difference in your accepting it, as I assure you it makes none in my willingness to part with it. He is always giving me something or other. I have such innumerable presents from him that it is quite impossible for me to value or for him to remember half. And as for this necklace, I do not suppose I have worn it six times: it is very pretty, but I never think of it; and though you would be most heartily welcome to any other in my trinket–box, you have happened to fix on the very one which, if I have a choice, I would rather part with and see in your possession than any other. Say no more against it, I entreat you. Such a trifle is not worth half so many words.”

Fanny dared not make any farther opposition; and with renewed but less happy thanks accepted the necklace again, for there was an expression in Miss Crawford’s eyes which she could not be satisfied with."