I left my MM15vn with 4 20Rs in it before I left for my deployment last April, and I just got back.
It seems the cells are drained to 0v as they’re unprotected and the MM15 doesn’t have low-voltage cutoff. Checked them with a multimeter to confirm, and yep, zero volts.
So are the cells now garbage? Or are they revivable at all?
They are damaged, do not use them anymore at all.
recycle them
Garbage 
Check gearbest they have had some deals on 25Rs in the last days around 13$ for 4 if you want a cheap replacement but beware they need some weeks to ship them so if you need a fast replacement that is often no good option.
I was thinking thermite or laser beams… 
Have you done these cells a favor NO
Are these cells “toast” well we don’t actually know.
Is it safer to replace them YES
Could it be interesting to see the exact condition of these cells (well maybe?)
If you have a low amperage charger (hopefully one that will also provide an IRR reading). Set them up out side and try to charge them up slowly.
Think about it, you have four dead cells (by definition no juice in them) if you can charge them for a few minutes and see the IRR change to or stay in a safe range then it may make since to keep charging them. If they stay cool then one can keep going with the “experiment”
The cells can not go in to thermal runaway if they are cold. (but it can happen fast) I had a cr123 cell blowup (aka violently vent flame) in less then two minutes once, (but lets just say do to extreme cell abuse I was not surprised that it vented) The final stage of the charging cycle is the time most likely to get “overly warm”.
and you may want to only use the cells in lower amp applications in the future. So for this light go and get new better cells, but these might might still be usable cells for more normal applications.
Will the cells be like new (probably not) were they toast before you put left them? Were they new? Was the flashlight stored in the freezer? was it stored in the oven? it can make a difference.
Are replacements cells cheap these days, YES, are better cells available these days YES. Do you want to make an experiment to see if these are usable again (up to you).
I would say “Do not”! put them on a cheap 1A Chinese dumb charger on your bed covered with pillows and leave the house for the day!!!
But out side, and monitor the beginning and ending of the charging process especially well, might be an easy way to confirm that exact condition of those cells.
They were brand-new when I got them a couple years ago, weren’t used much as the MM15vn was mostly a shelf queen. Stored at room temperature.
Is there a safe place to recycle/dispose of them in Germany?
Conrad Electronics stores do have recycling-containers for lithium batteries (at least in Dresden they do). Maybe you check it out if there’s one nearby…
I’m heading to Media Markt tomorrow - I’ll see if they have something. 
You can put them in any battery recycling bin, almost every store has the small little green boxes.
I used the box on DM for my bloated lipo cells I had, they have also containers for energysavingbulbs.
Officially there are yellow boxes for lithium batteries available, but I have not seen one. So I just taped the electrodes with some PvC tape and threw them in the standard green box.
Check this document for the theory:
http://www.grs-batterien.de/fileadmin/fileadmin/Downloads/GRS_1-12-0071-01_Basisflyer_Sichere_Erfassung_105x210.pdf
They say 10% Lithium Cells is okay in the green Box.
My question is, if the cells were protected, would the protection have kicked in at low currents, I am not even asking about a few dozen microamps, already at 1-2mA wouldn't be low enough to skip the protection from kicking in and continue to drain the cells?
Anyone tested this aspect on various protection circuits?
Li-ion protection circuit normally have several sensors, namely overcurrent, overvoltage and undervoltage protection. Any of those gets outside the allowed value, it trips.
So even in low drain current, it doesn’t matter. Once the voltage drops under the treshold, it will trip as undervoltage.
Actually on a couple of occasions I have read about the protection not kicking in... so that is why I ask, possibly bad circuits.