@diver I felt much the same but just kept coming back at it with breaks in between, solving another clue or 2. Once I'd started to enter things it did fall into place reasonably quickly. There are many entries that aren't affected by the two letter/cell thing.
The fact that there are the same number of 3 letter positions in the grid as there are answers tells you something about the 4 letter ones. I'd solved all of those and the SW corner provided a 'way in'.
Looking back, there's also a hint or two earlier in the thread about the NW corner.
Once the central 9 letter one yields you are pretty much home and dry. 👍