Thanks, this is definitely very useful. I presume I am more of a hands-on person, try this and that, bang it, bend it, shim it and make it work one way or another or decide no such action can succeed. In the short-term I will limp with what I have and look whether no other unforeseen problems emerge. In the meantime I carefully straightened the arched stub in the K frame mount and managed to position the derailleur lower on that stub, hopefully stressing it less. If the setup turns out to be stable over time, I will look either into a printing such as yours or just get the respective pieces brazed on onto the seatpost, in the end saving weight and complexity.