I also found the NE and SW corners harder, particularly the SW. I was so stuck in the SW after filling all the rest that I did something I haven't done in a long time - I hit the 'autofill' button in Sympathy just to see what it might come up with, and went through some of the possibilities to see if they were supported by the clues.
I'm inclined to agree with Hagen about the jumbling of the replacements, though preamble info did help with some. I found three locations easily but struggled with the last two, especially as I was missing an extra letter for one of them because the wordplay of one clue completely defeated me. In the end I saw a location for the other letters so managed to deduce the missing letter.
I must say that I did admire many of the clues, though there are still four that I don't fully understand.
One more task remains - the highlighting. Nothing's jumped out at me yet.