I have the theme, the completed column and row and the thematic pairs. But, typically, I have failed to find some of the incorrect letters and can’t yet make a sensible instruction.
The one I can't parse is 18d. I found the definition in Chambers, but ...
Am I not seeing it because I'm American? I've looked on the map but to no avail.
As you've filled the left column and most of the bottom row you'll know where the 14 answers are positioned in the grid. You should see a common feature in some of them (then look it up in Chambers, keeping in mind the bottom row).
30d - the "flower is not botanical
31d - look up Thetis and set her realm upwards (facing Olympus,) with c for caught inside
37 - the key is a commonly used computer key + 3 letter position with 1 letter for king inside