I have successfully removed LE´s invincibility via mistform in my mod. Gkrit, your welcome to include it in your mod as well, I am pretty sure I documented all the changes that need to be made. If not, you can compare code to determine the exact changes needed to be made.
In my mod, the mana cost was slightly reduced, his evasion was raised to something like 40% to take into account he loses the ability to autoattack while in mistform, or use any other abilities for that matter.
So, when he mists, he is immune to debuffs as before, and they get removed. He can evade 40% of regular attacks but does not seem to evade ability damage. He is no longer invisible, so he can be attacked without see invisible items. Although he might be slighty harder to target because I kept the mesh graphics intact. (He is in mist form after all)
It ends up being a nice support ability as it does damage, makes him harder to kill (but not invincible) and increases health regen of everyone around him, him included (Again to account for his disadvantages of not being able to autoattack or use other abilities while misted).
Now, if not used right, it can be a death sentence to him. Because he is vulnerable and unable to move for around 3 seconds while he is in mist form and while he shifts back to normal form. Makes it alot more challenging to use and nowhere near invincibility but hopefully is still a nice ability for atleast killing creep waves and supporting your own minion army.
Should probably not be used if being focused though. Check it out.