I am not sure why fogged highlights would be mottled. Mottling is more a problem with the paper PH or with it's humidity. One thing that would probably change if you leave your coated paper sitting around is it's humidity.
One reason I have never tried to adjust contrast by adding stuff to the developer is because it seems to me it would leave questions about what the developer is doing. I adjust contrast with Pt/Pd by variously using hydrogen peroxide or Na2 or classic part B.
Another thing in my experience is that the more platinum you use in the mix, the more likely you are to have mottling or grain problems. I keep my platinum content down to 10-15%. It only takes a little bit of platinum to improve the density of your blacks over pure palladium.
You can make a print with quite a lot of ambient light but I work in my basement and use 25 watt lights and if I leave a coated paper sitting around awhile I will either put it face down or on a dark shelf.
Try putting 1 drop of hydrogen peroxide in your mix to see if it clears up your highlights.
Dennis