Actually, I was able to reproduce 1) and 3) in a reliable manner.
Let's ignore 2) for now, which happened just once or twice.
If I boot the laptop and directly start X it works. I get as a user to
a working X environment, including windowmaker, etc. I can exit X,
re-start X11 again, it works
When I boot, I do not have network configured. I insert into the
laptop either a LAN or a WLAN card and run manually dhclient on it.
Once I do that, I get into the "X starts and gets black screen" forever.
Interestingly, logging in remotely and killing all X related process
(which do die) doesn't bring back a console, a black screen remains.
This looks quite weird.