I'm really glad you liked Fern, she is one of the more complex turtle, mechanics-wise.
Now, onto the bugs:
1. Thanks for reporting the flask interaction with overheal. I've implemented a general fix to how consumables are handled, which should indirectly resolve this issue.
2. Regarding stamina regeneration: it doesn't actually drain stamina when you're overhealed, it simply doesn't activate. The apparent stamina drop you're seeing is likely the cost of digging the cell that triggered the regen tick, not the regen itself.
3. I haven't been able to reproduce the issue with restore cells causing underhealing, but just to clarify: overheal is applied at 50% of the original heal amount. So, for example, if your max stamina is 50 and you're already full, collecting a +30 restore cell will give you +15 overheal (rounded down).
4. Similarly, I haven't been able to reproduce the underheal effect from titanium tools' "free dig" when combined with restore cells. It might be related to that same 50% overheal rule, but let me know if you spot a consistent pattern.
5. For the Sanguine Signet: the stamina bar shows the total change, so let's say you're at 46/50 stamina and pick up a +30 restore. You'll see a +17 change, that's 4 to top off your normal stamina, and (30 - 4)/2 = 13 going into overheal.
6. I have fixed the bug with headgears unlocking twice (hopefully).