Some update:
Game doesn't allow to heal the Crystal with Cleric's heals or Mystic's insta heal. It was possible by using certain hack that works only on PW 1.3.6, but it doesn't work on any newer version (we are running version 1.4.5). However in the mentioned TW indeed few Mystics were using Healing Plants to 'heal' their Crystal. But it seems that the heal from the Plants doesn't work at all and doesn't increase Crystal's regeneration rate. And even if it does, the heal from it 125HP/2 seconds is useless compared to the normal Crystal regeneration rate which is 2700HP/1 second (43 times higher).
If you have other opinion, you are free to test it with the next TWs and share your results. For now I will keep the thread closed.