While I agree on her intentionally postponing it, I reckon the real reason is that she enjoys the current pace. This ought to be her first romance, just like it's his, so she's simply taking everything out of every phase of it. It's naturally made possible by the (unrealistic) Saitou/Hiiragi situation, but since it's possible, why not do it? She knows they will become a couple of later, so unless she gets impatient, there's no particular need to hurry. He won't suddenly find someone else, which might be a real danger in a normal situation when approaching someone you haven't had that much contact with or who hasn't even considered a romance with you. They are both literature people, so they aren't impatient by nature.
I feel like when he actually finds out, it's gonna be a big plot point. Cause think of how shocking, embarrassing, and maybe even humiliating it will be for him to find out that all the stuff he thought he was sharing with a neutral third party was actually the other person? So maybe that'll become a future drama.
Nah. I've said it before, but the whole situation is so unrealistic that there's no way it would happen in RL. So, you can't also judge it in terms of what would happen realistically afterwards. I mean, if he was offended by Saitou keeping it a "secret" and humiliating him, shouldn't Saitou also be deeply insulted? She's supposedly the girl he loves, but he can't even recognise her if she dons glasses. You might as well say any girl who reads books is fine for him, then. So, no, I don't expect the author to make any drama out of it. Maybe a single chapter of embarrassment.