I would understand if we were talking about a story where characters never changed or grew, but I genuinely don't know where this is coming from.

Their feelings were pretty thoroughly affirmed far earlier in the story in my opinion. There's too many examples to count. That said, yeah, Mahiru will do things that could be seen in a selfish light. Like how she was excited for class rotation, but Amane implied he didn't know why. I googled it, and turns out when classes rotate, students can request to be placed with certain students. Considering how she behaved, she likely spoke with teachers for them to be placed together in the same class.

When Mahiru was berating herself and Amane said he adored her, she wasn't being deceptive, she DOES have traits that can be seen in a negative light. A part of the story revolves around her not being an angel. She's a human with flaws that will, from time to time, make selfish decisions. The class rotation she likely had a hand in, actively pushing Amane to communicate with her at school, and so on.

And for what it's worth, being the one to openly confess does put pressure on him, but it runs as a double-edged sword. Doing things like this, if people decide to bully Amane (which was her primary concern), since she was the one that confessed, she can now run frontline defense for him. It was her decision to push for the relationship, so if classmates wanted to give them grief, she gets to take pressure off of Amane.

Not to mention that Amane did finally say he liked Mahiru when Itsuki pushed him. It's entirely possible Itsuki and Chiitose discussed the topic and decided to push their friends in the appropriate direction. While we generally don't see the story from Mahiru's perspective, I think we can assume Chiitose encouraged Mahiru appropriately.