Detail is always good, don't worry
There's a thing called a UT/OT loop where you get a UT nap and then because of the UT nap, you subsequently get OT. What a pain!
Seems clear that getting A time right has helped him. I'm wondering if he needs slightly more than 1 hr 30. Would you try 1 hr 35, or 40? Rather than patting, you could just lay a hand on him, or try a soothing noise. If he's mantra-ing, then I'd say you're doing the right thing to leave him. Listen, and if he's calling for you, go and help him, but if he's happy enough, then leave him to figure it out. You're right, he will get it, and sooner rather than later, most likely.