And another dull day begins.

We won't truly know till Act 3 comes out but, based on his behavior, I strongly believe he will choose to stick to his plan over staying with us. He shows discomfort and hesitation at having to go back to finishing off the other symbols after our second date, but he still chooses it. I think his mentality now is something along the lines of, "I've come this far. I've waited for this for so long. This is the only option. I'm doing it."

Even if he loves us, the only motivation he's had for most of his prolonged existence was becoming free from the pain of such existence. So at this point he's adamant that destruction of everything back to nothing is the only way. No questions. The way he erases his memories with Duae to numb any doubts emphasizes this. He's willing to make personal and painful sacrifices for this plan.

So. To actually answer the question, I think he will "betray" us in the sense of choosing destruction over us in the end. But it isn't exactly betrayal if you look at it with emotions detached: you agreed to his deal, and that deal is allowing him to use us for his means. It was shaken on, with your knowledge, from the beginning.

But, I strongly believe he won't be satisfied even after his plan is successful and everything goes back to nada. I'm unsure if he will even succeed in achieving peace from the pain of his past lives, because erasing the garden itself doesn't exactly equate to the erasure of his existance, right? (...I'm skeptical of his plan because we don't know the full specific details and mechanism behind this whole "delete world>delete me>delete my pain" process. But I'd still choose to help him, regardless of if it means a goodbye. I wish I knew more about his plan...)

However, I believe Nulla will regret and lament over what he's done/what he could've done/whether this was the only way. Especially as his plan likely goes south.

At least a little. Maybe a lot. Maybe only feel regret and remorse.

... If I'm being honest, I can't easily think of a true happy ending for him and us. The concept of a being self-aware of its fictionality and limitations, craving its own erasure, but loving us... It's a tragedy from the beginning as I see it.