Hi Wispo,
glad to hear you like my mod

Its a pretty hard change you suggest, since making "Inner Sanctum" a passive skill will prevent any spell-based character from picking it, as long as its toggable, you can use it occaisonally, even as a mage. Metamorphosis is after all a more interesting mastery for melee characters, but this doesnt mean casters arent meant to use it. So the easier solution for your problem would be to make the skill undipelable (yes, thats possible), so noone can remove it from you, altough it has no cooldown nor an activation cost, so dispelling shouldnt be a problem.
If you want, i can create a modified version of achilles_destinymod with an undispelable "Inner Sanctum" and send it to you, but releasing a new Version of the mod for only one single Change which isn't taking care of a game breaking issue seems inordinately.