Got it now!
I didn't figure out 1) that launchpat cannot launch ./myts.arm, 2) that I had to install the usbNetwork hack to access the shell and 3) that the usbNetwork howto is only described in its bundled README_FIRST.txt
I understood correctly that the Ajaxterm is not needed. The launchpad cannot be used to start myts, but does not conflict with it. More schematically:
1) copy keydefs.ini, launchpad.ini and myts.arm in a directory
2) install and activate usbnetwork from
here, following instructions in README_FIRST.txt (don't be too scared

)
3) telnet or ssh from the host, navigate to /mnt/us/<your myts.arm directory> and launch .myts.arm &
4) shift T on Kindle and you are done
Sorry for repeating things already known and thank you all for the great job and the support.