... I am getting mottled highlights even when I cover the coated paper with foil. The degree of mottling seems to coincide with how long the coated paper sits out suggesting it may be sensitive to spurious UV in the house.
Potassium Dichromate shouldn't cause any highlight problems unless you are using way too much of it. And then it's less of a mottling and more of a sort of flat graininess - a bit like if you had over-brushed the paper. Are you sure that you are using a 10% solution of Potassium Dichromate? The proportions that you've quoted (2ml of 10% in 1L) are what I (and many others) call 'Grade 2'. This is fine and doesn't cause any problems. I have seen problems when I've used 30ml or more of 10% per litre of developer and this has been coupled with excessive drying.
What is your developer temperature? This effects all sorts of things, but so long as it's in the normal range then it shouldn't be the cause of your problems.
You don't have enough Platinum in your sensitiser to see any graininess/grittiness from it. You only really see this when you're using Potassium Chloroplatinite on its own without _any_ Sodium Chloropalladite. I used a 50:50 mix for years with Potassium Oxalate/Dichromate, and nowadays use an 85:15 Pt
d mix. It's only with 'pure' Platinum that I've seen grittiness.
Three other thoughts occur: (1) As Dennis posits there may be something strange going on with your drying process; (2) You may be damaging the paper when coating it; and (3) There may be something wrong with your Ferric Oxalate.
First I'd confirm that the problem isn't in your Ferric Oxalate. This is easy to test by mixing a sensitiser solution using fresh Ferric Oxalate. One, probably daft, question re. your Ferric Oxalate: you are using Ferric Oxalate #1 aren't you? If somehow you've got mixed up and are using #2 then the high level of Potassium Chlorate in #2 will definitely cause mottled highlights.
Assuming that you haven't changed your paper, paper storage method or your coating process (sensitiser volumes, brush/rod, times, etc.) then I would discount that for now. If you have changed any of these variables then I would go back and test these with your old process.
That leaves us with drying as a potential root cause. Can you describe your drying method? How are you drying the paper, and for how long? What is your ambient temperature and humidity? Do you have constant ambient temperature and humidity or do you have draughts or other variables? And while you're at it, what paper are you using and how much sensitiser are you using?
Could you post a scan of the mottling so I can see it? Oh, and another dumb question: it's not that your paper is sticking to your negative is it? You should know if it is, but a quick check of the back of your negative should show whether there's anything sticking to it.
One last thought. Even though this doesn't sound like fogging on its own, perhaps it could be fogging in combination with another factor that accounts for the unevenness. So it's worth being aware that platinum/palladium sensitiser is sensitive to visible light, so if you are fogging your paper then it could be any light that you have switched on. So are you drying in the dark or with lights on? And if the lights are on then how bright are they?
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.
Just for the record, Dennis, Potassium Dichromate works in exactly the same way as all the other restrainers (Potassium Chlorate, Na2 solution, etc.). It inhibits the laying down of Platinum and Palladium by converting Ferrous Oxalate back into Ferric Oxalate. There's nothing mysterious going on