What is the correct Islamic course of action when dealing with female attendees at a wedding who come dressed immodestly, knowing that adherence to modest dress was stipulated in the invitation cards? And is providing blouses at the door considered coercion?

It is permissible for women to remove their abayas if they are alone in a place safe from the sight of men, provided that this does not lead to the exposure of their 'awrah. However, it is not permissible for a woman to expose any part of her 'awrah in front of other women, and she must be enjoined to desist from that. If she insists, she should be asked to leave, unless a greater harm is feared. As for someone who is highly likely to expose herself and not respond to advice, it is not permissible to invite her, as that would be a contribution to an evil act.