Manual modification of a PSF is usually a bad idea.
Yes this is where I was having trouble. I had underestimated the sensitivity of the psf file when I was adding my cross-terms. The alignment of the numbers is apparently significant and I was not aware of this. I had been trying to keep the same quantity of delimited spaces between each column instead of trying to keep the numbers in the columns aligned.
I attached an image of the correct alignment that I am now using.
This was not very apparent in my OP as the forum reformatted the text in the !NCRTERM block, though it is visible at the bottom of psf.txt file... but this made all the difference. CHARMM no longer crashes with the MISSING PARAMETERS error.
Thank you @rmv for your help.
