I now tried some random experimental koken versions on the heavy bag - with power - for the first time ever and my conclusion is:
1. koken uchi in this form works great
2. Intuitively the upward version of the same should word fine, although its harder to train on a straight punching bag hanging down - the hunch is that this is the most powerful version; where I can put more power in using the chest muscles.
3. I also tested then downward "oroshi koken", pretty uch like starting out exactly like "shuto mawashi uke" but I angle the arm in the end and strike with koke instead of shuto.
4. I also improvise with a what one may call "mawashi koken", so insstead of mawashi seiken tsuki, I angle the arm and strike with koken. But this was at least for me NOT good! When you apply power, it leaves your elbow in an akward and vulnerable angle. So I would say not robst, but high risk of damaging your own elbow joint. It seems the twristing of the arm, destabilizes the elbow. Or it was my experience.
=> The similar mawashi haito I regularly do at the bag, and even in sparring, but then only to the upper arm, or maybe armpit. But the angle of koken makes in unstable.
The hunch is that version 2 will give more power, 1 and 3 are comparatively weak, but they will of course do for head strikes, but not for body strikes I think. 4 is bad for me, will not do again.
the most practical version seems to me to be for example this combo