Why is Stephen even invited to the wedding? And Lucy constantly looking for him and staring at him is embarrassing

Obviously we have more episodes to come, but I don’t understand why anyone wants Stephen at the wedding, let alone Evan and Bree who haven’t liked him much at all.

And Lucy could not be more obvious that she is constantly on edge and looking around for him, like she’s not fooling anyone. ETA: I’m aware that Lucy is traumatized and I definitely feel for her, but I’m saying she should be a bit more calculated in how she lets that be expressed. She plays it cool with basically everyone insisting she’s fine, but then still darts around and shifting her eyes to find him constantly. The hot tub scene was insane of her. She’s playing herself as a clown instead of being smart and pretending (a bit more believably) that she doesn’t give a fuck.

Lastly- why/how is he with Lydia?! Like just to make Lucy mad?? And double down on ruining her life?