I see (:D) but how is JA circumspect with Cassandra about her reasoning or is suddenly formal about Mrs. Knight’s proposed gift in my interpretation? I don’t see a lack of frankness towards Cassandra rather I see in the service of gratitude, kindness and consideration a great lack of frankness to Mrs. Knight. I think JA felt and wrote to Mrs. Knight with kindness and gratitude but jokes with Cassandra about how the gift is unwanted and unneeded.

The paragraph from Ltr. 74 dissected as I read it:

I have taken your hint, slight as it was, and have written to Mrs. Knight, and most sincerely do I hope it will not be in vain.

Jane wrote to Mrs. Knight at Cassandra’s urging because the lady “threatens” to give Jane her spinning wheel. I can believe Jane “most sincerely” hopes writing to decline the gift will not be in vain for Mrs. Knight’s sake and her own. I think accepting the wheel comes with an obligation to put it to good use—something it appears JA sincerely wishes to avoid because she has no wish to spin. How many times have you been asked how a gift is working out?

I cannot endure the idea of her giving away her own wheel, and have told her no more than the truth, in saying that I could never use it with comfort.

I can also believe JA did not wish Mrs. Knight to deprive herself on her account—especially as it appears the wheel is of no value to JA beyond the kindness and generosity behind the gift. The phrase “and have told her no more than the truth” suggests there is truth JA did not reveal to Mrs. Knight about why she could never use it with comfort. I think the truth spared is that JA would have no pleasure in the gift.

I had a great mind to add that, if she persisted in giving it, I would spin nothing with it but a rope to hang myself, but I was afraid of making it appear less serious matter of feeling than it really is.