As the chapter opens, Gene says that he "wanted to see Phineas, and Phineas only." Why is it so important for him to see Finny at this point?

I think the heart of the matter is that Gene needs Finny more than ever. Gene's emotional state is fragile after the Leeper confrontation. Gene has built his identity around his dysfunctional relationship with Finny. Now that everything is in chaos, Gene needs his Finny to hang on to. He needs Finny to validate his own identity again. Yes, it's on the creepy side.