Not very much worth saving? Reinstall?
That reinstall is going to be tricky if you cannot get some form of boot going on.
My first cut at this is recovering the mac mini is a serious priority, but not the highest priority.
#1 Priority is to give yourself a pat on the back for wrecking your computer. I mean it. Everyone is terrified of wrecking their computer. You have done it. It is a bummer now, but if you recognize the significance of this situation, you will gain more from this than you could have gained during the time it is down had it stayed up.
The other #1 priority is to learn the two crucial things that will prevent this from happeing to you again. I mean,with any luck and hard work, you will wreck your computer again, but that is not a problem. It is a hassle. The problem is you don't have a handy way to recover your system.
ioannispol, I want to tell you something. I urge you not to remember who tells you this, but what I am about to tell you is something you now have a chance at never forgetting and by never forgetting, you will have a shot at never putting yourself into the horrible position you are in of having a wrecked computer with no recovery means. This is a mega foul up. It is good to do this... once.... if you must. Now you know that taking steps to have a recovery means available is something you have to do to avoid this sitaution.
Everyone wrecks a computer sooner or later. It would be better if no one did but I was not consulted when the ways of the world were established.
So with that as the introduction, the first thing to learn here is to never forget to make sure you have a way to recover your computer and never put yourself in a position for this lame condition to become what you are now again.
The second thing to learn is, how to have a way to recover your system.
The short answer is: you need a bootable drive for your computer that you can use in the event your existing bootalbe drive dies. Your bootable drive died. You need another drive to boot it now.
I am not sure what your options are . You can get thumb drives to boot that mac mini as a mac. You can get a thumb drive to boot that mac mini as a linux zombie. The linux option might give you a better shot at fixing your Tweak thing or recoving your files if you have any on there. If you cannot fix the Tweak the starting over could be a good move after recovering your files.
You can think about this a lot, but your options come down to about those.
p.s. They don't have to be thumb drives. Any bootable drive you can connect to that machine will do, probably usb connection,not sure if fire wire provides the right connection for booting.,.. it probably does, I just am not sure.
Never be without a recovery disk again.
Good luck man.