Creating multiple profiles under one user to assign to particular posts

Sounds to me like “Profiles” should be a custom post type with custom fields. You can then relate the profiles to the users via a custom taxonomy or even more crudely by just storing the Profile post ids as meta data of the User. This would give you the flexibility of offering multiple profiles for a single user.

Creating a form on the frontend would then capture their changes and match it to the custom fields of the Profile post.