Is “in so unpleasant a manner” actually grammatical in English? Why?
Solution 1:
In so unpleasant a manner means in a manner that was so unpleasant. It's quite a common construction in English, as an alternative to in such a... The writer used it because that was their stylistic choice.
"You shouldn't stay out of doors for long on so cold a day." "You must be tired after so long a journey." etc. etc.