Thank you for your valuable thoughts!
My consideration was:
A low roll bar (like the road version of the Librands) is better than no roll bar IF the risk of unprotected banging the head against the bar can be prevented. For protection I would mount the headrests on the upper hoop.
Am I wrong with this consideration?
So I went for a tall roll bar split in two sections, the lower half is permanent, the upper half is fitted when needed. It isn't the prettiest thing, but does the job it was made for.
What is the difference between this solution with the lower half and the Librands road version?