Thank you for addressing these!
2, 3, 4. This bug is still present; I'll try to explain it better, haha:
When simultaneously regenerating and collecting restore from a low enough state, the regen stacks on the restore cell with the 1.5 multiplier, -1 for the dig itself. This is correct. When regenerating from an already overhealed state, the regen does not add any bonus, but the next dig costs -1. This is also correct, even when simultaneously collecting a restore cell while already overhealed. The problem is when regenerating upon collecting a restore cell that would have otherwise caused overheal.
Try to collect a restore cell that would cause overheal (maybe with 32/50 stamina; this should get Fern up to ~56/50). Next, try to do the same thing, but get the restore cell in such a way that your regen kicks in at the same moment you dig the restore cell. This will cause underheal (stamina goes up to 56/50, but regen takes it down to 50/50 instead of taking it to 55/50 or even leaving it at 56/50. 55 should be the correct outcome, because +24 overheal/-1 dig and no regen when already healed). The status update on the stamina bar actually shows -7 for this specific example, which seems like the regen is added to the overheal bonus, but all 7 stamina points are removed. It should instead show -1 and go to 55. The same issue was happening with the titanium free dig, but I haven't been able to check if that was fixed (requires luck, haha). The issue is still present when wearing Violetheart; the stamina will go up to 56, then 58, then -8 back to 50.
5. The Sanguine Signet bug was actually that the (30-4)/2 part was not being calculated consistently when below a depth of 50 with Fern; it would vary by some amount. I haven't tested if it has been fixed yet, so I apologize! However, Sanguine Signet's effect is still affected by the simultaneous regen bug