>Mienshao uses Drain Punch on a level 100 Blissey with 252 HP EVs. Assuming Mienshao OHKOs, Mienshao then U-turns out on the next Pokemon, causing Regenerator to take affect, all while holding a Big Root.
Blissey's HP is 714 with full investment.
Drain Punch recovers 50% of damage dealt, so 307 HP normally.
Big Root increases the amount of health returned by 1.3x, so it becomes a 65% drain rate. This is equal to 464.1 HP, or 464 HP.
Mienshao's maximum HP is 334.
So, regardless of how many HP EVs you're running on Mienshao, you will return to 100% if you OHKO Blissey. There is no need to factor Regenerator.
So, since Mienshao's HP is capped at 334, the most you will heal will technically be 333 (Since if you're under 1 you're obviously fainted). If you're only running 4 HP EVs, then the recovery is capped to 272 HP.
>Also, how much health does Regenerator restore if Mienshao is at level 50, with no HP EVs?
At level 50, Mienshao's HP is 140. Regenerator restores 1/3 of your HP, so 46 HP, rounded down.