Hi malone and thanks.
I think you're right about taking both ends off HOLLOW (and then overlooking the H to give me THE for a start to the overlooked letters, which makes for an odd clue).
I've got the theme by looking at what overlooked letters I do have. And sort of an instruction, although the wordplay for 27a seems to give nothing useful.
For 12d I have ?AE?ER. I'm fairly sure an extra L is in the wordplay but no word seems to fit what I have.