Your first idea worked like a charm, Arcalane! This is what I did:
I made A fake attachable head that does not move and I put the heavily damaged one behind it. It bleeds and all but the actor does not take health damage when the part is hit. However, I did a little searching and read about the BreakWound string. So when the part is destroyed, the heavily damaged head is revealed and the actor starts to lose health rapidly, dying seconds later.
The krogan still fights during this so it's pretty awesome to see it fighting till its last breath. xD
The real head is smaller, and I hid it the best I could behind the actor but you can still see a couple of pixels when the actor is going prone or has been knocked back but it's just a detail that can be fixed.
I added a 1 in EmissionDamage and now the fake head takes damage when it bleeds so it's even better now.
Also, I made them really heavy and added a bit of Sharpness to their body parts so they can kill by jumping over actors or if you jet-propel forward and hold down the arrow key, they will do a headbutt attack that will knock someone's weapons sometimes or just kill them if there's a wall behind them.
Oh man, I had to increase the gibbing resistance of its torso even more... explosives gib it and it makes them drop all its heads, lol.
I also made its heavily damaged head unable to be hit so it doesn't expose the helmet hole. The real head is hidden in there so if the Krogan somehow survives the fatal breakwound, a bullet that goes thru the area will hit the real head and kill it instantly.
Dammit, explosives would still gib the torso area and the only thing that seems to prevent it is the missioncritical string. The problem with that is the actor will never settle when activated. I tried some lua thing to deactivate it when the actor is dead but I can't make it work.
I tried attaching the heads into a single one but having the GetsHitByMOs = 0 in the main object prevent the attached ones to have that setting to 1.
I also tried to make a very durable material for the torso with high gibbing resistance and structutal integrity, AND I also made a fake torso in front of it that works like the fake head, so when it break, the actor will get the fatal breakwound. But even with all this, the entire torso would still gib, releasing the fake heads.
I gave up on the indestructible torso but I found workaround. I managed to set up a lua that gibs all but the heavily damaged head when they are detached from the torso. Phew...
I'll do some more tests and pixel editing when I get back home from work and if everything is working ok, then I'll probably release this unit as a mod.
Hmm... I made a helmet for it but maybe it would make it too OP:
It still needs some pixel adjustments to make it fit better in the slot.
The helmet turns it into a walking tank and it can make the krogan survive even a rocket to the face.