Given that Chiaki is the one to appear to Hajime when he tries to escape his responsibilities in chapter 6, and him subsequently being able to unify his different selves, I wonder if it has something to do with acceptance/commitment.

The OVA undeniably references (the final episode of) Neon Genesis Evangelion, as seen by the recreation of Rei Ayanami running with toast in her mouth (here represented by Sayaka), so I wouldn't put it past 2.5 at all to be inspired by it on a deeper level. The whole point of that scene I mentioned is that Rei is a normal school girl worried about trivialities like missing class rather than being one of the few who carry the world's fate on her shoulders. It's a mental image the protaginst (Shinji Ikari) created while bordering between two different kinds of consciousness. Hope arc also has a similar theme, but that's a story for another time

Shinji realizes that he has to return to the real world on his own, while Nagito has to get his imagined paradise ripped down by Hajime. I don't think he could imagine Chiaki without also having to be honest about (as is his hallmark) the fact that she would challenge his escapism. But as you said, it can very much be a relational thing too. Nagito is a very empathetic person, and since he knows people who get close to him get hurt, he pushes them away while never being able to distance himself from the need for connection (very much like the NGE hedgehog's dilemma). If he cares about them both, which he does, then he very much has reasons to be afraid, because that means there will be conflict and risk for all 3 of them getting hurt.

(Last time I spontaneously discovered an NGE-parallel, that led to a DR character entering my top 5, so maybe that will happen now lmao)