If you want your pulled pork to turn out great, it is going to take longer than your electric smoker.
I typically do not soak my pork butts before cooking, but I doubt it would hurt. The mustard binder and seasoning are great. You can use anything you like for seasoning. Some suggestions are: Brown Sugar, Sea Salt and Pepper, Onion Powder, Garlic Powder, Dried Mustard, Smoked Paprika and Cayenne Pepper to taste. Personally, I love apple cider vinegar flavor on pulled pork, adding it later in the cooking process, but I was raised in Virginia where such vinegar based sauces were common.
I suggest setting your cook temperature no higher than 225F/ 107 C. Be sure that is the actual cook temperature as measured a few inches away from the pork. Do not rely on the controller reading. Traeger recommends a cook temperature of 250F, but I like cooking it a little slower.
Allow the pork to smoke for several hours until the the internal temperature reaches the stall point (165F/ 74C). At this point, remove the pork butt from the smoker and wrap it tightly in heavy duty aluminum foil. Alternatively, you can place the meat in roasting pan and then cover it tightly. This is where you can add some liquid such as a cup of water, apple cider vinegar, or a good English pale ale. Once the meat is wrapped, it cannot absorb any more smoke, you you can finish the cooking process in either the Traeger or even in your kitchen oven.
Allow the pork to continue to cook until a final temperature of 203F/95C is reached. Remove the pork from the cooker, keep it wrapped and allow it to rest for a couple of hours to cook down so you can pull it.
Since you are doing this cook to impress your in-laws, I would strongly recommend that you do the cook a day or two ahead of time if at all possible.. Cook timing can be highly variable depending on the size of the pork butt and other factors. The last thing you want is for the guests to show up while you are in a state of panic waiting for the protein to come up to temperature. That is a recipe for disaster. Fortunately, pulled pork is very forgiving. You can keep it in the refrigerator for a few days or keep it for several months in the freezer. It will be every bit as good when reheated as it would have been had you just finished cooking it. Thus, let your in-laws believe that you have been up since the wee hours of the morning preparing their meal. All the while, you can be relaxed knowing that the meal will be served on time.
