Looking good to me - you are well on the way to some fine pulled pork.
To answer a couple of questions all at once.....
I usually fire up a chimney of charcoal for my WSM and then start the prep of my shoulders. Only takes a minute to trim the thick fat off and apply rub. By then the chimney is ready. So I don't let my shoulders sit around at all before smoking and they are usually in the 35* range when they hit the grate.
The rule as I understand it is to get the meat above 140 degrees in the first 4 hours unless it is chemically cured. More important for ground meats as you have a lot of surface area potentially exposed to contaminants during the grinding and shaping process. With a solid uncut muscle like a shoulder it's the outside surface that matters most not the core, so time is less of an issue.
As to 18 hour smokes, sometimes it takes longer. 95% of the time I can bring in 4 eight pound shoulders in 18 hours at 225* in my WSM. Last week it took 22 hours so go figure. Sometimes the meat wants to stall and you just have to wait it out or bump up the temps a little.
As to probe placement, I probably would have used a middle rack shoulder and would think in a LP smoker the lower rack may tend to run a bit hot. But you would know your smoker and how it performs better than I as I'm using a WSM and not a LP model. By my calculations you should be about 15 hours in on the smoke now, if you get this post in time, take a reading in each of the shoulders and see if they are all running about the same. Don't sweat it if they are off a little as shoulders are very forgiving. It's also not unheard of to have to pull shoulders at different times based on internal temps. Depending on who you ask, anywhere between 195 and 205 is good to go (I usually pull mine at 200).
And like Nrok2118 said, foil them and let them rest in a cooler for at least an hour (IMO two to three hours is better, but that's just my opinion and not based on any factual testing). The internal temp will rise a little and the juices will redistribute. Then let the feasting begin....