I like it! I've been playing around with the idea of a shapeshifter mage for a while (themed as the Howl, since they are already canon), though as Nature/Dark rather than Nature/Mind. The problem I've always had is why you would want to shapeshift rather than simply summoning the creature directly. You address this, though:
All enchantments that target a mestaclocan instead target their active form. All damage that would be dealt to a mestaclocan is instead dealt to their active form. When one of their forms takes damage equal to or greater than its life, that form is destroyed, and the Mestaclocan reverts to their original form, which is incorporeal. A mestaclocan can only really die in their original form, which has 10 life.
I think this is a nice incentive - the creatures you become are your armor. So while you have fewer actions, you are extremely hard to kill and can be very unpredictable (once you have a lot of creatures, your opponent could be facing anything next round).
In fact, you might want some disadvantage to make up for the incredible ability to survive and prevent the mage from just loading up on 'meatshield forms'. Since this is a Mind mage, upkeep costs could make sense.
On the other hand, 10 life/incorporeal is a pretty big disadvantage once the creature is stripped away, since there is no way for you to heal, and you can't shift again until the next round. Maybe it is fine.
I'm not sure if Blink is the best secondary ability for this mage. Also, as has been discussed elsewhere, Blink X > Y is not necessarily always better than Blink Y, since you might return to the zone that you left and have less control over your destination. Thematically, I can sort of see the Mestaclocan appearing randomly in different places, but maybe there is something better you could do.
Edit: Just saw your post on why blink works well with nature/mind mages. Sounds good, though you may want to be more explicit about the thematic justification.
A few questions:
1. You said that equipment only applies to the original form. If you are shapeshifted, can you still use your equipment? If not, then I assume your equipment is also not targetable, right?
2. Can you still choose to summon creatures normally?