Jif, I don't think it's giving too much away to say that solving the wordplay from the four extra words was what brought enlightenment, along with carefully considering the meaning of
all of the penultimate sentence of the preamble. I didn't count the clues and therefore didn't notice the discrepancy, and I'm not very good with symmetry so that didn't help me at all. And yes, I had solved a lot of clues before realising what was going on, but I'm not sure that's all that relevant.
One solver has managed to complete the grid without understanding what the four extra words lead to (at least at the time he/she posted). I would never have been able to do that, but obviously it must be possible.
Any more than this really does need to go on the other thread, I think. I hope this is of some help (and doesn't overstep the spoiler mark).