Sounds to me like your thermo was outta whack. The center is normally hotter then the sides but not by 200* and not the other way around. I don't see how opening the vents could drop the temp with more air getting in.
Did you have the hickory on top of the lump or buried? I think you want the smoke early in the cook not later.
Not sure about shutting the vents down before taking the lid off. The fire was getting starved of air then you gave it a rush of air. Isn't that the backdraft type thing that happens in building fires?
So to your questions
2. I think you did the backdraft thing
3. put the wood on top to start with
4. if you keep it at 225 you should not need more cooking time the meat is plenty far enough away from the fire in a UDS
Drumhead - UDS x 2 & UDG
Keep on Smokin'
KC native missin' KC
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