1) I used heavy walled 1 X 2 rectangular steel tubing as a MDF straightener (circled in white) where the originator used angle iron. My table was more warped and I thought this would be better, but also I had scrap. It is attached with some stainless counter sunk 1/4 inch screws shown circled in green using some holes already present in my top. This did a good job but in hindsight I wish I had gone full width. My approach, for the obvious reasons, was to stay between the T-tracks mounted on the bottom of the table top for adjustment.
2) I added counter sunk drywall-style screws, circled in red, to serve as adjustable levelers for the router plate. I used two per corner. My plate fits well below the table top and this works better than shimming methods I had tried. Kreg sells some corner levelers that would probably work well, but I wanted to look at them before I bought and the stores that sell them in my area were and are limited to curbside only at present. (Kreg's table uses these and has no ledge, so modification would be necessary.)
I'm really satisfied and happy with how well this system works. I do still plan to add an OAR fence as I mentioned in an earlier post. I will add that info when I finish it.