Hi Novice, I will help you as far as I can (after staring at the grid for about 15 minutes yesterday afternoon, and again for a short while late in the evening, I've parked this for the time being. We're only here for a short time and it seems silly spending any more than a little of that time looking at a full grid to find embedded words).
Now it's a nice sunny day, so again I have the excuse not to proceed...
...but. If the 3-word phrase is the only way we can distinguish between the options for which letters to enter on the 2 diagonals, then surely it must intersect at least one cell with a clash.
Have you considered that the source may be R&J, down in the bottom right corner ?