Yeah, I read through all 70 pages, and no one seems able to explain Culber's change of mind. He was in full "I'm not in love with you" mode for months, and while that 30-second pep talk from Reno was nice, it doesn't explain the sudden "you're my everything" switch.
Obviously the writers are just putting people where they wanted them to be. Instead of technobabble it was romancebabble.
Culber didn't know what to make of himself, he needed space, and Stamets wouldn't give it to him because he wanted to be with Culber every moment. Culber in this whole other place, feeling as if he shouldn't even be there, pushed Stamets away
hard, so Culber would leave him alone.
Stamets was in this whole other place, and then he picked a fight with Tyler, hoping to bring out Voq. That didn't happen but Tyler got the anger out of his system that he had. And, with Stamets staying away from him, he finally had the space he needed. He got his space, he got his anger out of his system, then he was ready to reach back out to Stamets.
Stamets was so hurt by Culber pushing him away, that he rejected Culber wanting to make amends and would leave even at the sight of him. So now Culber was ready to kiss and make-up but Stamets wanted nothing to do with it, leaving Culber in this awkward space. But then Stamets felt in this awkward place too, because he didn't like being pushed away and he didn't want to be hurt again. He was worried he'd be hurt again if he went back to Culber again. But he couldn't get that off his mind, so Reno noticed he distracted Stamets was. Stamets also probably wasn't exchanging barbs with her, so she could tell if something was wrong.
That's when she went to Culber to say to work something out. Culber, who as I said already got his inner-demons out of his system, figured the best solution was to just transfer to the Enterprise. To told Stamets and wanted closure. Luckily, Stamets was able to see that Culber was leaving to make him feel better and they reconciled.
Then, Culber realized he couldn't do it. He couldn't go to the Enterprise. So he stayed on Discovery. Then, as Stamets was injured in the Control's attack, Culber healed him and they fell in love again.
I'm not really on this board to talk about the Mushy Stuff, but there's the explanation.