After a lot of searching I stumbled on a couple of postings which referred to the "Level Horizon" problem.
It turned out that the attitude estimator could be manually started from the MavLink console by using the command "attitude_estimator_q start" and this solved the problem. Without knowing the details I assume that this process should be running from startup because a manual start seems to be just a work-around and not a normal and documented procedure.
The third problem "no baro sensor 0" is still not solved. A check of the devices does not show any /dev/baro0 or similar device but I am not aware of how this should be set up so it is just a guess and I am not skilled enough to dig into the code find out. As mentioned I have found a couple of cases (but not for a Pixracer) where a change of firmware has solved this problem but my tests of that has not helped so far.
Does anybody have a clue what might be done solve this or firmly establish that the baro sensor is broken?