That's plenty of time but I suggest you take a motorbike-taxi to the border from the bus station instead of a Songtaew.
Also when you enter the Myanmar side you just tell the immigration officers "no shopping" and they will stamp you right in and out the same time. Turnaround at both borders should take no more than 30 minutes so you have time to grab some lunch as well at the busstation.
Just to double check because this is new info to me. You can still get you non-O extension after the original visa is expired but within those last 90-day entry leading up to the 15th month? Does this also count for non-B?