I've been hanging here a while and as you suspect I've never heard of anyone with that issue. Some thoughts :
The timing chain broke so it's possible that thing is timed incorrectly. If the slave camshaft valves make contact with the piston during the upstroke, the master camshaft (the one driven by the crank) will continue turning and proceed to break the small chain.
Coolant in the head area is alarming. I haven't been inside the M54 in a while but I don't remember the VANOS assembly being directly cooled, hence the coolant (if it was indeed that) is coming from a cracked head.
Some here have successfully timed their M54s, although I do remember that involves special tools. As far as procedure, even car "vets" have confessed messing things up with the M54. Experts like jbeurotech will have a better (real world) understanding. Hopefully he's watching and will give you an idea.