I am a newbie to the X5 world having only owned by truck for one month. During the first month of ownership, I had to address many of the same issues you mentioned.

I had the "Self Level Susp Inactive" and found there was some corrosion in the plug for the self leveling control unit. The control unit is located beside the battery (passenger side) and you have to remove the plastic cover beside the battery to find it. The control unit has the connector on top so is susceptible to corrosion if any moisture drips down from above.

I was trying to identify other electrical problems and pulled the suspension control unit plug off only discover corrosion on a few of the pins in the connector. Pin 15 was not making a good connection and it is used to send control information. When I fixed up this connector, the message went away.

This might not be your problem, but in my case it was the fix. With the "Check Brake Lights" and "Check Rear Lights" messages, the twist connectors on the bulbs for the tail/brake lights were slightly corroded and were fixed with a piece of emery paper to clean the contacts.

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Rick
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2005 BMW X5 4.4 (Highland Green - Beige interior)
1978 Porsche 928 (Copper Bronze - Cork interior)