[Enzeder]

Supposedly D mode is more efficient. But I suspect that depends on the driver. The theory is that in B mode regen still has losses which coasting does not. I suspect it's hard to modulate 100% accurately so you're not occasionally dipping in to regen in B mode.
I'm in the middle. My only prior experience in an EV is with the Bolt and there was no coast. If you lifted off the accelerator pedal in D mode the car would simulate engine braking - similar to an ICE vehicle. If you entered one-pedal mode (which I only did using the paddle), the nice thing was the car would come to a complete stop as smooth as a chauffeur stop - it was great. As far as I know there was no blended braking.

I've not adjusted to B mode in the ID.4 yet - I find it overly sensitive. And because the D mode uses regen whenever possible I may not bother training myself.

On the Bolt, the brake lights only came on past a certain threshold. IE: they did not come on when regen was used to simulate engine braking - which I think was about 15 kW. This would be similar behavior to an ICE car. However, if you hit the paddle and regen increased the brake lights would turn on - for obvious reasons.

Back in the day, if you wanted to slow down quickly without alerting cops behind you that you know you're speeding (or be an ahole), you'd pull the handbrake as there was no brake switch attached and the brake lights would not illuminate. Obviously not prudent behavior.