Suspension confusion

[primeral]

Hi all, I just bought a 2009 X5 35d. One week into its ownership I started having a sagging rear suspension after leaving it parked for a few hours. This morning I got the "Level control system malfunction!" message. Both left and right sides sag, so I'm preparing for news that something more serious than airbags need replacing. I tried searching but my keywords yielded no results, so here are my questions:

1) Do the conversion kits allow me to run the full Eibach 2093.540 kit? This is the one advertised to exclude self-leveling rears. Are there kits that incorporate lowering?

2) Are X5 and X6 suspensions interchangeable? I cross referenced Bilstein dampener part numbers and they are the same, but I wasn't too sure about spring heights and upper shock mounts / perches. I'm entertaining the thought of fully adjustable coilovers such as the K-Sport that's advertised to fit only the X6.

3) The fully adjustable coilovers that I do see (KW and K-Sport) show a coil spring over shock setup. Will I be able to eliminate the rear airbags and use that setup instead?

4) If I do abandon the airbag setup, is it difficult / expensive to eliminate the level control system error messages?

Sorry if this has been gone over before, I just wasn't able to find it. Thank you very much in advance.

Al
2009 X5 35d, Alpine White
2002 M5, Alpine White
2006 M3 SMG coupe, Alpine White