Thanks for your notes of the legal position. This reads to me like 12,000ft legal limit without O2 on board, as you say it only says I need a bottle. But this reads to me that if I go above 12,000 I do not need O2 unless I am there for 30 minutes or more, but I need it on board to bust 12,000. Is that a correct interpretation?
No, you need oxygen after 30 minutes at > 12 000ft. The law doesn't say what kind of system or that you actually have to breathe it. Once you climb above 13 000 ft, you need oxygen on board. If you don't have any and cruise at FL125, you have to descend after 30 minutes.
Sorry, in Germany when flying VFR above 5000ft amsl or 2000ft above ground it is MANDATORY to maintain a Flight Level according to the semicircular rules (55,75,95,115 from 0 - 179 degrees magnetic track, 65,85,105 etc from 180 degrees to 359 degrees).
In reality nobody cares about this. I have never been asked by ATC to respect the semicircular rules and I regularly fly even levels (IFR) or other direction level in airspace C (which implies ATC clearance to do so). In my understanding, this is more a historical thing back when radar coverage was limited.