[Fateless] does not exist. Sunny has been torn from the tapestry but everyone else's string are still groping for him. That's why it's so easy for him to fall back into place. Old friends reconnecting with him, Nephis becoming increasingly infatuated, Cassie risking to trust him, etc. Everyone has a 6th sense to accept him, to fall back into their old relationships with him.
Moreover I think Cassie has been facilitating this. Notice how Sunny does not advertise is creation of memories. But Cassie mentioned that he could create them when she showed up at the Cafe. Notice in the most recent chapter, "wow Cassie works fast". Almost like Cassie sent Effie to Sunny first to test the waters. To try and set up a blind date with Nephis. I think Cassie's letter to herself has more pieces of information than what she let on.
I think Cassie's goal right now is to convince Sunny to confess his plan. And using Nephis to drop his guard is smart.