Mileage is only of relevance to VW Finance if you hand the car back at the end of the term, as they will charge you extra based on a p.p.m figure, though it doesn't work the other way round!
You should NEVER simply hand back the car at the end of a PCP, unless your personal circumstances make it unavoidable. You will always do better by paying off the balloon and then deposing of the car by your chosen method (sale or p/x).
Not always. My wife has a Smart ForFour and the balloon is £6500 as of December 2018, the car is just about worth that now so likely to be worth £5500 at best this time next year. It will be handed back without doubt. I agree that this may be an exception to the rule as I've never done this before, although I have heard it's quite common with Mercedes as they set the GFV high and depreciate a lot.
That's a fair point, but most of my experience of PCP's has been with VAG cars and I think they tend to give realistic future values. I suspect Mercedes up the GFV in order to get lower monthly payments, which is often what attracts buyers more.
I have always been able to sell cars at the end of the their term for more than the balloon payment, sometimes by only about £1,000, but once by over £6,500 more (Skoda Yeti). Usually I expect to get between £2,000 and £3,000 over, but I generally treat my cars well and make sure they are well presented for sale.