Borrelli, thank you for detailing the disassembly in the other thread. Though, it's a bummer that this requires removal of the top and fronts as well - I don't like working with glued-on parts, because re-attaching them after removal is never working well for me. I have no idea where to find the proper glues here in Germany. I used to try several glues I got at hobby places (where they sell self-assembly miniature planes etc.), but nothing worked reliably. I may just save me all the hassle and send it in, though 90€ is a high price when a complete body is now sold used for less than 250€.
But enough of my whining. I know what my options are now. And it's now also clear that the C ring ("circlip") from uscamera is a plastic piece, more likely to break in the future again. Finding or constructing one from metal might be the smarter idea for anyone going down the DIY path.
Rick_B's foam solution post is here, BTW:
https://www.pentaxforums.com/forums/115-pentax-k-5/189715-lens-release-button...ml#post2967996. It assumes that you still have (or purchase from uscamera, see
http://uscamera.com/77240-a107.htm) the original button part, while you won't need the circlip nor the rubbery cone for his solution, if I understand it correctly. It's similar to gluing the cone to the button and to the camera body, but less messy, I suspect.