I really dislike all black interiors so would give up exterior colour choice for a better interior. Most sporty cars have dark headlining and plastics so it's even more important to get some colour to avoid it being like a cave.
1) I drove it for around 7 months with the open differential, starting in January 2018 so including some winter driving. It's not that bad if you leave traction on or in Sport. I don't do lurid slides everywhere so the main difference is better traction off the line and when powering out of a corner. With the open diff you could feel the inside wheel spin up when it gets unloaded in medium speed corners, which led to a loss of forwards traction but the car quickly regained grip, albeit with a bit of snap. With the Wavetrac or a Quaife you get that beautiful sense of connection between your right foot and the rear end. The more you gently apply power the tighter your line.
I got the Wavetrac fitted by Regal Autosport in Southampton as they are the UK dealers. It's £1,600 drive in/drive out. The basic principle is the same as a Quaife but it has a wavy ring to still transfer some torque across the axle under light/zero load conditions (wheel in the air/on ice). It also has carbon fibre bias plates to maintain a constant friction level over the lifetime of the unit, where the Quaife wears the metal housing smooth over time.
I had a Quaife fitted to my previous car which was a Z4 Coupe and I'd say the Wavetrac feels slightly more aggressive in it's operation. It does seem to pass more power to the outside wheel in scenarios like pulling out of a junction and going over the crest in a road which lightens the inside wheel. As the price difference is minimal over the Quaife (if at all) I don't see a downside to choosing the Wavetrac.
Wavetrac® Differential - A torque biasing differential with a difference
2) There is a an improvement in ride with the 18s over the 19s but not so much as to not want to run the 19s in better weather. The 19s are from the 205 C63 and are 0.5" wider front and rear so perfectly matched to 245 and 265 tyres compared to the 235/255 standard setup. The offsets push the rears out by around 4.3mm each and the fronts by 13.4mm each, which still fits within the arches.
3) The seat isn't pushed all the way forward. My wife can sit in the front without hitting the dash and she's taller than 5'2". If I sit there my knees are brushing the dash and I'm 5'11" with long legs. It is quite tight but not a deal breaker and it should improve as they switch to a forward facing seat.
Be careful choosing a car seat as most don't give the baby a reclined enough position, so their head can flop onto their chest. We ended up with a Cabriofix on a Family Fix base. A lot of seats didn't work, especially those designed to last for longer as they grow. The problem is the rear bench has a slight recline but then so does pretty much any car including family buses like the Zafira etc so it's a car seat design problem.
). I'll see how things go without an LSD (my wife will definitely never notice lol) and then make a decision from there.
