Would an heir of heart be the one to develop a persona, or have an idol, roleplay as them so much that they start confusing the persona/idol's character with their own? They'd need to be really careful not to lose themselves completely.

I feel both Witch and Heir of Heart could definitely be at risk of losing their Self indeed, whether it’s a persona, an Idol, or someone else, however, it’s hard to say without specific examples.