Why on earth would anyone want to fill the arches? Think about it. If you really did that, when you were steering right or left the wheels would hit the arches every time you went over the tiniest of bumps. The gap is there for a purpose: to allow the excellent Mercedes suspension to do its work. It is because of the suspension travel that it doesn't have the awful ride of an Audi or a 5 series.
You need to consider not only the diameter of the wheel and the profile of the tyre. As the wheel diameter increases, the tyre profile decreases to keep the overall rolling circumfrance the same.
The 19inch wheel and reduced tyre profile over a 16inch wheel and higher profile should not rub the arches, if Mercedes say a 19 inch wheel and 30 or 35 profile tyre fit in, they fit. End of.
I however like you, would say 19 is getting on a bit and would prefer an 18 which is still blooming massive, or a 16/17 on a lesser engined model (as the extra grip afforded by the wider wheels is not needed, nor are you likely to drive in a manner where the additional "wheel control" from the reduced flex you get with a lower profile tyre a factor needing considered).
However, on a larger engined car equipped with sports suspension you may well drive more progressively and the extra wheel control afforded by less tyre flex as the profile is reduced, as well as the extra grip, in the dry, may well be worth investing in.
By the way, I have driven the 212 with all the available wheel sizes up to 18 inch. The larger wheels greatly increase road noise. And spoil the true Mercedes ride. They also reduce the feel of the steering due to the greater resistance to turning of wide low profile tyres. They also aquaplane more easily, have less grip in the wet, and increase fuel consumption (see data in the price list). It is only the little boy in us that wants to look like 19 year olds in a Nova. Logically the bigger wheels make no real sense. And look naff IMO.
As discussed you get greater control with the larger wheels, and I note, from personal experience that the larger wheels give a lot less grip in the wet, and probably marginally reduced steering feel.
However, fuel consumption I get from the larger wheeled biggered engined 211 is about the same as my last one!!!
Only driven a 212 on 18s and the ride was subliminal. This is one major improvement over the 211, the ride is a lot better. As is the steering and cabin. Where it falls down is the seating?
Aside Hawk as you know the dealers well, would they be able to retrofit the seats from a E class coupe into a 212 saloon, I have a back issue and found the 212 seats would hurt my back, but the coupe seats are marvellous. Oddly 204 seating is ok?
I'm really beginning to believe that the average British male would now ideally like solid alloy wheels with no tyre at all.
It would save me £800 every 30,000 miles or so for tyres. Maybe 35,000 at this rate. They are barely worn. Remarkable. However, £800 is a lot of money