Thanks for the hints everyone. I couldn’t have completed this one without your help.
It was fun to learn a few new words along the way, and I really like the tight logic of (most of..) the clues. I will look out for more of Picaroon’s work. I am interested in science, so his knowledge base fits well with mine.
I agree Chrise that “knocked back” for 21d would be more appropriate for an across clue.
I think 16d is more than a straight anagram, more of an envelope/anagram combination, because four letters are switched around and put inside the others that are also jumbled up.
27a was LOI. I thought there was only one word that could fit, but that was a blind alley. Again, happy to learn a new word