On an M12 X 1.5 thread, it needs 15mm of thread engagement to achieve maximum clamping load capacity. You'll find that with spacers, you probably have about 12mm of thread engagement, which isn't really enough.
The ECM puts the glow plugs on a lot more than it puts the light on. The light only comes on if you need to wait for the combustion chamber to pre-heat for a cold start. However the ECM puts the plugs on even if they're not needed for starting, as they also improve combustion.
As above. The offset of post 2001 rims was changed to accommodate the larger brakes fitted to the later models.
As suggested, it's best to get the correct rims. A 5mm spacer should move the rims out enough to clear the brakes, but the studs aren't really long enough to allow correct engagement...
You don't have to drain the the granny, but be aware that pulling the IID out will allow a reasonable amount of fluid to spill out.
Measure what comes out, and replace it with the same amount of fresh fluid, which will do a partial change.
The rear fog lights are controlled by the CCU, as the rear fogs must stay off after the ignition is cycled. The front fogs don't need to stay off after an ignition cycle, so can use a latching switch. The front fogs were also a dealer option, so were supplied as a kit of everything needed (I...
It's probably worth getting the injector balance checked using suitable diagnostic equipment.
An injector that's not firing correctly will cause an imbalance, most noticeable at idle.
The FL2 would be more reliable than a BMW, and only a bit less reliable than a modern Toyota, which aren't as good as older models.
Nothing wrong with the FL2, except for the Haldex, and even those can be fixed.