JA's fainting on hearing the news about their removal to Bath could be a result of nothing less than the severest of shocks ever, because we know what she thought of 'fainting'.

I can well imagine that as suggested, the letters written in December were the ones where JA gave full vent to her feelings of anger and grief, because while reading letter 29 I was struck by the cheerfulness therein and JA being so much her usual self.
But I think there is an underlying current of feeling.
It may just be in my imagination, but I think when she starts the letter by saying;

"I begin with the hope, which is at present uppermost in my mind,.."

It's kind of an understatement of her real feelings and what is really on her mind.

The above statement sounds a bit like the one about her gown on which the principal hopes of her happiness rested.