Hi there, I thought I'd add my 2 cents. My DS is 15 mos, and our story sounded *a lot* like yours!
My experience has been that a NW that's an hour after he first goes down is classic overtired sign. And then the NWs can continue from overtired.
They only way I was able to get out of this cycle was to a) start Wi/Wo to teach completely independent sleep. and b) move bed time back pretty significantly from 7:30 to 6:45.
I found that using Wi/Wo got me out of the lingering gradual withdrawl/I'm a prop game and using it in the night made the NWs that used to last for hour/s quick, 2 second things.
And, I found that moving bed time earlier made it possible for DS to get enough sleep to end the overtired cycle which was causing that first 45 minute mark NW, and in our case also a 5:00 AM wake up call!
Hope that's helpful for you!