i appear to have solved the problem ( less by knowing than by trying...): i booted into ubuntu and when fully booted shut down ubuntu. after that i could boot into OSX. in OSX i went to preference "startdisk" and "reconfirmed" my main OSX boot disk again. after doing this it now appears, the boot ram is back to routine again as booting via "alt" shows all available boot devices. ( i remembered something in the context of refit about "reconfirming" boot devices in startdisk even so it appears redundant to do so) so re-confirming the main OSX boot device in startdisk may become necessary once a linux or other OS has been booted via cd and may have left some imprint on the bootprocess. hope fully functional status of the little machine stays as it is now.
further comments: no linux installed on the machine, no extra bootloaders, etc.
observation along the problem regarding recovery partition`s diskutility not working on SSDs which also hold the main OSX system:
after booting into ubuntu had worked and rebooting into the main OSX disk was possible again i booted into recovery partition and attempted to run disk-utilities on the main OSXs partition. disk-utility got stuck and could not finish, also did not produce any error messages. makes me wonder if recovery partitions on the same SSD as main system do not work. possible internal conflict in the SSD ?
i had to run disk-utility via an external disk to make it run completely ( it did not find any errors on the main system). then i ran disk warrior on the main system as well. it noted to have repaired some volume information on the system disk. still it appears that "re-confirming" the main OSX boot device in "startdisk" preference was the step to normalization.
this was my first post on this forum and a great experience, seeing others trying to help. my sincere thanks to all who tried to help and contributed !
message to apple support staff:
recovery and downloads for all other firmware editions appear to be on the site but not the above version. this leaves owners of the respective mac mini in danger of not being able to re-activate their machines if the bootware and firmware get corrupted.