Sounds like you learned your lessens well. I don't know if standing more is one of the lessens I would take away from this. It is much easier to be still when seated which I think we all would agree is important. Sometimes the deer get through the lanes before we see them and there is nothing that can be done. The only answer to that is to see them earlier. So are you scanning the woods often enough.
You may have missed an opportunity on the buck by not shooting through the small stuff at fifteen yards but maybe not. Your instinct at the time was to not shoot and I find it is best to follow the instincts and not second guess yourself. Split second decisions are what deer hunting is about and my guess is you made a good one not to shoot.