Appreciate the feedback from everyone. All was not as desperate as it seemed. Yes, I did end up with some jerky edge point, but had about 2 cups once cubed to chili-ize...
Other than being a little dry, most of the flat was actually good. It wasn't ready once I noticed my error, and being more uniform, there was only a little waste around one side (that had been cut in processing and I didn't notice while shopping), I think I got a decent hang, and it was pretty consistent considering. I wrapped as soon as I noticed, which was after 3.5 hrs. The flat went back on wrapped with some apple juice added for maybe another 2 hours at 250-ish (after the drum cooled) until it was probing tender, temp was around 210. It rested in the oven that I occasionally switched on to keep it around 100-150. I ended up slicing the flat when it was about 160. I wonder if I had just let it rest in the oven w/o toying w/ it then sliced at 150 it would have retained some more moisture.
I monitored the temps closely. The majority of the cook took place between 365-380. Only near 400 once and at the beginning. I was only getting ~10 degree spikes when opening to spritz. My inexperience led me to not know what was a "set" bark, and I'm still not quite sure. It did rub off some when being grabbed with my gloves, definitely not majorly adhered, but as you can see by the slices, it isn't just falling off. I assume being large ground pepper, it's not going to turn into an encasement, and will still come off somewhat easy if rubbed? Or is this a result of the exterior edge drying too quick? For lump vs. charcoal...I fail to see, scientifically, how 400 degrees is different from 400 degrees? I can understand it's propensity to spike higher when exposed to air, but I didn't notice that in my cook. On the flip side, I've been running into a problem where after 5 or so hours, my bricks begin to choke themselves w/ ash and I have to shake them loose a little (releasing an undesired plume of ash).
I think for the next cook, I'm going to baffle with a large roasting pan full of water, and cook w/ a temp probe in, so I can monitor the goings-on (this was my first time ignoring it...whoops). And maybe I'll aim for 350-375, as I was trying to have it ready for dinner, not lunch...If you read this far, thanks for taking the time and assisting.