A royal diplomatic visa means no US taxes and only UK taxes, I believe.
Not UK taxes, either, if he spends less than a certain amount of time in the UK.
The problem for the House of Windsor is that the monarch has to sign off on that particular piece of paperwork. That means at least QEII and possibly Chuck.
If so, there will be hell to pay.
I’m betting the visa and the reasons for granting it are why Deep States on both sides of the pond are covering for the Harkles.