I've already put my entry into the mail with a completely filled-in grid, but two things still don't work out for me.
One has already been mentioned: I can find only one plausible answer for 17ac, but I cannot figure out how to justify the wordplay in Chambers. Other dictionaries do list the needed abbreviation, but if Chambers does as well, I haven't found where.
The other concerns 44ac. There are two unchecked cells, and even after figuring out where the other five letters go in the jumble, I can't see how there could be any possible way of settling which of the remaining two letters should go in which unchecked cell. Therefore, if I'm interpreting the preamble correctly, after we have filled in as much of the grid as possible, those two cells are still empty. Then we follow the instruction obtained from the across clues, and when we're done with that, those two cells are still empty.
(To be honest, those are not the only two cells that are still empty for me after filling in as much of the grid as seems to me to be possible. But with the rest of the grid, it may be that there's a subtle logical path that I just haven't found yet; with 44ac, it seems evident that there cannot possibly be
any chain of reasoning that will make it possible to decide which letter goes in which unchecked cell.)
Yet the preamble says that, by following the instruction from the across clues, we are "thus resolving or overriding the initial ambiguities". But here is at least one ambiguity that, at least as far as I can see, is neither resolved nor overridden. At the same time, it seems perfectly clear what letters
should end up in those two cells. I just can't see how following the instructions gets those letters into their cells.
So the preamble itself seems to me to contain a sizable ambiguity that I'm unable to either resolve or override. As I said, I've already submitted my entry with every cell filled. I reasoned that if the correct answer involved leaving some cells empty, surely the preamble would have to refer to resolving only some of the initial ambiguities.
Are others seeing something here that I'm missing?