Dome temp lower than grate. What gives?
Man with 2 clocks never knows what time it is. 
I think some of that principal applies here. I don't know the answer, other than stating the obvious - calibrated differently or wrong. I'm sure 2 different locations in the smoker could result in different temp readings. Just pick one, learn it and how to cook with it, and carry on.
OK... So here is the recap thus far:
I'm not trying to be a pain-in-the-***, but I'm going to breakdown your post and add my $.02
Started at 4PM U.S. EST. I have essentially had to replenish the WSM with hot coals/mixed with unlit coals every 2 hours. After the initial cool down, the temp stays at a steady 225 for about 90-100 minutes. At the 2 hour mark, the temps have consistently fallen to 200 degrees F. I go through the process of lighting additional briquets in my chimney and then adding them to the WSM. The temp again stabilizes to right around 225. (After a few minutes)
I gather you're using regular ol Kingsford which is commonly known as KBB (kingsford blue bag.) How did you get the WSM going? As in, how much unlit and lit did you add to the ring in the very beginning? Are you solely judging your temps via the dome thermometer?
The amount of Kingsford I am using seems a bit excessive. I have already used one of the bigger bags and started into another. It is a bit breezy and the temps are hovering right above freezing, with a few flurries. I am thinking that the breeze has something to do with it. I have closed the vents to about 25% or so at this point.
When you say bogger bags, do you mean a 20 pound bag which is part of the twin pack sold at HD/Lowes? If so, that's A LOT. Yes, wind will cause more havoc on these smokers than anything else. But not enough to burn through THAT much charcoal. I didn't even use that much charcoal when I had 65 pounds of pork butts on my 22 WSM which ran for 14-16 hours. Clearly, there's something else going on here.
Having to add charcoal every two hours - is this about normal? I am using a full chimney of lit and about a half chimney of unlit each time. (22.5 WSM with water)
No, 100% not normal. If you start with a full ring of charcoal, adding coals at the 10-12 hour mark would be normal. Could vary some given the colder temps, wind, desired pit temps, amount of meat, and water in the pan. But nothing even close to every 2 hours. A full chimney of lit and a half chimney of unlit? Seriously - that's what I use when cooking a turkey at 350-400 degrees. Which brings me back to the question of what your pit temps actually are. (not what the thermo reads)
I took the ribs and smaller tenderloin off at the 6 hour mark. After letting them rest a bit, I tore into both. They both are very tasty (though the smaller tenderloin and part of the rack was a bit dry)... The real test will be the pork butt. I am shooting for a 10-12 hour cook on that.... I have no thermometer/probe yet, so I am sort of winging it for my first run-through. My Maverick 732 will be here on Tuesday. Couldn't wait that long to try my WSM!
It's good to know how to cook without the aid of the Maverick, but I also believe cooking with one makes a guy a better cook. So, I'm glad you jumped in feet first to the deep end of the pool.

If things seemed a bit overcooked/dry to you, I'm betting it's because of the pit temps. Seriously, there is no way you can burn through that much charcoal trying to maintain 225 degrees. As for the pork butt --- do you have a skewer, ice pick, something similar? If not, a regular ol' fork will also work. When the skewer goes into the butt without resistance, it's done. If you need to 'push' it through, it ain't done. Same with the fork test - stick the fork in to the bottom of the tines and twist. Should be done with nest to zero resistance. Plus, now you have a sample to taste. lol
Update: At my 2 hour check at just before midnight - happy to say that the temperatures were still rocking at around 230-240 (or so). Whatever I did this last time seemed to have worked better. (Essentially added a higher number of briquets)
What's your overall stats now? How long as the cook been going on? How much charcoal burned? Any other info so we can assist for cook #2 would be great.