about PP's claim. Well yes if you put the open movement to UV lights all day long then your oils may dry but that means that you need to wear a short sleeves while wearing your watch and keep your arm in a way for the watch to be exposed to UV rays during the whole day which is pushing it a bit too far. Furthermore this is also valid (and even more) for skeleton watches but I have yet to hear about oils drying up so I'm not totally convinced about the reasoning.
To answer your other questions:
1) I'm not totally sure that tourbillons actually improve the accuracy of a watch, according to FP Journe putting a tourbillon in a watch is like breaking one's leg before a race. Tourbillons were originally created to even out the effects of gravity in pocket watches which were only kept in one position. This no longer applies to wrist watches and tourbillons actually may have a negative effect on accuracy which explains the great difficulty in regulating a tourbillon wrist watch and what can set apart a tourbillon from a leser brand and a Tourbillon from lets say VC
2) I'm personnaly not a fan of COSC for modern watches where techniques enable watches to perform at COSC standards or even better and my understanding is that VC tourbillons have a rate accuracy well within COSC standards. I'm not aware of future plans of getting COSC certificates for their tourbillons.
All this to say that even though tourbillons have absolutely no real necessity in wrist watches they demonstrate the technical savoir faire of the few brands who can actually make and regulate one properly.