Having gone to bed some time after midnight on Friday, confident that I would manage to polish this one off after an hour or two more effort, I find myself having made precisely zero progress since Saturday morning. Embarrassingly, a block of four contiguous clues is proving completely intractable, and I don't really understand why.
As yet I've found no ambiguity which needed to be resolved, which is slightly worrying. I have the quotation, its author and the thematic word, and only two letters are missing from each of the thematic phrase and the instructions, so I'm pretty sure I know what they are. They in themselves tell me that the construction cannot be run-of-the-mill and that the final PDM will be worth the effort.
I hate relying on thematic devices to reverse engineer the grid fill, but I've got nowhere in the last day and a half.. Maybe analysis of the positions of the omitted letters will shed some light on the obstinate clues.