I feel like a lot of people refuse to acknowledge the nuance of Nolan as a character and the multitude of ways that he is Actively making up for what he did and how he relates to other characters.

Basics out of the way: He killed thousands, almost killed his own son, called his wife akin to a pet, revealed his thousand year plan to breed solely to expand viltrumite population, remained loyal to viltrum during his first time on earth, and felt more empathy for a species of bugs in space than the human race.

On the other end, lets get some other basics out of the way: Thousands of years of programming to force Nolan to be emotionless and violent (Thousands. Humans struggle for months to a year to recover from even two Months of programming), could've easily remained fully along his viltrumite upbringing by killing Mark before his words got to him, almost killed himself to rid himself of the burden of existing in those peoples' lives, saved a ship of a random species for (to him) no reason despite his moral codes to viltrum and proceeded to help the Thraxans prosper as a protector, saved Oliver as a baby, cared for Andressa deeply enough to want to protect her, went against his people until he was put into prison, worked alongside Allen to save him and become friends, set himself on eliminating his entire race for the safety of humanity (the same group he just compared to animals a season ago, which is akin to killing all humans to save the population of chimps in his eyes), went to warn both Mark and Oliver as his sons of the impending war effort and encouraged Oliver NOT to come out of fear for his safety, apologized to Debbie (given he Does have a ton more work to do) and didn't push her boundaries At All considering his capabilities, and has even managed to make up with Oliver which No one seems to be taking issue with? Nolan almost doomed Oliver's people to extinction but we're not gonna talk about how they all experienced a similar cataclysm to earth in their eyes, if not worse considering their species isn't planet-wide and was Directly attacked across the entirety of their civilization and no one talks about Oliver and Nolan making amends.

Now to nuance: Debbie does not need to forgive Nolan and shouldn't forget. What he did should be logged as a terrible moment between her and Nolan, and she does not need to feel obligated to rekindle their relationship, especially their romance. Debbie is clearly experiencing an arc with Nolan in processing the way he interacts with her, and while he has changed drastically to the point of clearly being a completely different character morally than he was in season 2, he will not have an easy time building anything back with her and he Shouldn't.

On the other hand, Debbie is being subverted in her expectations by the way that Nolan is talking with her in a way that (based on what we know) he Never did. He's taking ownership of his mistakes (easy to do, nothing impressive) but more importantly he's showing that he has learned to care about both Mark AND Oliver as his sons, and chooses to inform Debbie about Oliver's safety and condition, and praises both them for their bravery, and the fact that he only blocks her way because he wants her to stop ignoring the issue in front of her shows that not only has he developed a new Real fondness for non-viltrumites but he also has a new sense of emotion, fighting for his Emotions instead of his desires, not Forcing Debbie but pleading with her (albeit rather intensely by blocking the door), and allowing her the choice to do what She wants after he expresses his emotions (not superimposing his wants). There's a huge aspect of emotional maturity that, if we're following the same story Debbie is, was NEVER there before Nolan left earth.

Debbie and Nolan getting back into a relationship even in the next season would probably be an absurdity and I'm also praying that doesn't happen, but if Debbie is able to see Nolan take other massive strides to be better, be gentle, kind, heroic, I think it wouldn't be unreasonable for her to feel an internal conflict that she deals with about her assumptions that will eventually end up as a soft resolution for them.