I agree with both of you. JA may definitely have given Fanny the east room with this purpose.

And I also think it's interesting what BarbaraB wrote about the kinds of chains that were given. About Mary's (Henry's) chain it says:

"It was of gold, prettily worked; and though Fanny would have preferred a longer and a plainer chain as more adapted for her purpose, she hoped, in fixing on this, to be chusing what Miss Crawford least wished to keep."

Later when Edmund gives her his chain, it is exactly what she is describing here as the kind of chain she wants - a plainer, longer chain and more adapted for her purpose.

I also definitely agree with Christopher that Mary and Henry now appear to be collaborating. They may have been collaborating the whole time, and it might have been just a short moment of empathy that Mary felt for Fanny when she asks Henry not to plunge her too deep.

Actually, when Mary is trying to con Fanny into taking Henry's chain she even says as much:

“you suspect a confederacy between us, and that what I am now doing is with his knowledge and at his desire?”

I think this scene is very telling of Mary's personality. After Mary has finished her conniving efforts, Fanny gives in and decides to take the chain, and JA writes the following:

"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."

What is Fanny seeing in Mary's eyes? It's almost malicious and really gives me goosebumps.