Imagine Samurai with actual stances: Target Eyes, Target Throat, Target Spine, Target Heart. They would actually change the way you stand and hold your weapon like classic samurai movie stances (one held over your head, one held behind you, one held in close pointing toward the enemy, and one in the sheath).

Ex: Fast Draw: With no stance it would stun, one stance would stun and move you through the enemy, another would make it a gap closer, and another could make you jump backward after striking.
You could change between them with no penalty (.5 sec cd), they wouldn't combine, and you'd go back to standard stance after use.

Closer to AST than NIN, yes, but less risky since risk isn't particularly good for tanking, and full of potential for utility.

Their actual tanking/dps stances could be holding the sword with one hand or two, or a wakizashi you'd draw into your off hand for dps.

Mitigation would be enfeebling debuffs and damage avoidance like Shirahadori.