My experience has led me to believe that with nominal smoke chamber temps of 225*, the bark/crust formation doesn't really get going until the internal temp has climbed over the 120-130* range, so it's not very dependent on time in the smoker, though, the weight of the meat will effect this time, larger/heavier taking longer than smaller/lighter. If higher chamber temps are used, the bark will begin forming earlier, but this has alot to do with the internal temps climbing faster, as well.
Stabbing the probe @ 6 hours won't hurt anything...main thing is to not probe really early and possibly force bacteria into the center of the meat from the outside before the surface has been pasterized by smoke chamber heat.
If the shoulder/butt has a decent fat cap on, it shouldn't dry out inside...never had a dry butt, unless it was fresh, no fat cap, and I didn't brine it. I like to smoke fat-cap up so it self bastes to protect it from drying. You've got to have alot of negatives working against you before drying out will become an issue, so don't sweat that.
Being boneless, you don't have an easy gauge for shrinkage (bone sticking out), which indicates the cooking progress, so internal temps will be your best indication, along with creep on the cooking grate. Large cuts like this will shrink around the circumference, and as they do, they'll get taller, almost as if they are bulking-up trying to shrug-off the heat. Anyway, they will get taller and smaller foot-print on the garte, so you can see where the meat was laying on the grate and has pulled itself tightly together, using less grate space...it's just another indication on cooking progress, and any time I do open the smoke chamber, I look for this.
Yea, plan on a long night, and possibly being able to slip in a couple hour nap now and then (set an alarm?) when you're comfortable with everything. The long smokes will test the ability of a charcoal smoker, btw. Ash build-up can be a problem if the smoker hasn't the necessary mods, or wasn't built for the long runs. Also, how long it will run on a single load of fuel will reveal itself.
Be sure you have a full propane tank (if that's your type of smoker) or a couple extra bags of charcoal/lump (whatever you're using). I don't start a long smoke in my charcoalers without at least 60lbs on hand. Just a couple things to remember that some folks get snagged on now and then...yep, count me as one of them...LOL!!! Nothing sux worse than tossing a half-cooked butt or brisket into the dreaded "O" to finish it up....especially when an average packer brisket won't fit in the "O"...
If she really likes the crust, you'll be looking for a no-foiled smoke, so that'll definitely be a long run...maybe just tent the shoulder with foil in a pan while resting, instead of wrapping tightly in foil...a bit less self-steaming while it rests, which will soften the bark...the longer the rest, the softer the bark will get.
I'm guessing, depending on how steady you can hold chamber temps, with that heavy of a shoulder, you'll likely hit a plateau in the mid/upper 140* range, or possibly lower 150's, then, probably another in the low/mid 160's, so just keep the smoker happy and let it ride. I have had stalls hit again in the 180's as well...that's where alot of folks have reverted to foiling...thing is, the stall is where the natural tenderization process is occuring, so I don't like to mess with mother nature's work, myself.
Easy does it, which it sounds you already know, and it'll be a great pulled pork feast!