] A couple were not supposed to write to each other unless they were engaged, and it was a breach of decorum to do so. The Morlands turn a blind eye to it, so they evidently don't think that it's a serious breach,

I think it more that the Morlands considered Catherine and Henry to be engaged, just not "officially", pending the General's consent.

] But -- a clue! -- Eleanor has a means of undertaking secret correspondence in her friend Alice, and she is prepared to use it when she wants to hear from Catherine. Was Alice acting as a cover for Eleanor's correspondence with her young man?

But Eleanor had Catherine write to her under cover of Alice, as she was to be at Alice's home: "Direct to me at Lord Longtown's, and, I must ask it, under cover to Alice." The excuse to kick out Catherine was that the Tilneys had a prior engagement at the Longtown's. Eleanor asked Catherine to write to her under cover of Alice, because she knew the General would be keeping an eye on her correspondance.