Whenever we kept a bedtime of 6pm more than a few days, DS would wake earlier to compensate. It was a catch 22 at times. Not fun, I know.
At this age, the timing of the morning nap can dictate wake up time. So your plan to move out the nap will help a lot, I think. He might not be able to make it to 12:15 at first, but don't be discouraged... if you keep at it, he'll adjust... especially if his morning WUs shift later.
12.5 hr day is fine, but their morning and afternoon A times can alter the sleep rhythm quite a bit.
I've stopped by some of your threads before and I thought LO moved to 1 nap around 10 months? I remember bc mine started 2-1 around 9/10 months.