Yeah they don't do monotubes. There are pros and cons for both types of damper but Monotubes are better for track days as as they don't bend as much in the corners and also don't suffer from oil cavitation. Road cars don't tend to push dampers very hard, so generally always come with twin tubes as they're also much cheaper to make. KW offer a lifetime warranty on the inox V3s, or they used to at least, and you won't see that kind of warranty with monotubes
The main negatives of monotubes are harsher ride and shorter life span. They tend to feel the same across the brands because of how they work, except where companies like Ohlins use a 'blow off' valve to bleed off some of the high speed compression forces (riding apex kerbs etc) which gives you better wheel control. I think AST do something similar.
Bilstein and KW only get decent with their clubsport range, but you wouldn't like them on the road
Linear springs all the way
Progressives are unpredictable and inconsistent.
The GCs are the only mount I would consider. OEM BMW bearings for the springs, which last forever, but are only £25 each when the time comes....and for the damper rods they use fairly decent Aurora spherical bearings, but I upgraded mine to NMB bearings which are heavier duty
I think the cost of consultation and revalving your existing dampers might not be good VFM vs upgrading them to adjustables, but have some chats with race experts and see what they say
It never hurts and they tend to raise things you might not have considered.
My chat with BalanceMotorsport ended up with Gaz Gold coilovers running much softer spring rates than I would have used, but it worked really well. That's why the MK7 Clubsport S broke the ring lap record.....because it used softer suspension