I've never dealt with a picnic with a solution & I haven't injected so I can't shed any light there, but I will say that you should allow approx. 1.5 hours/lb. Now this isn't gospel truth, it's just a rough time frame to start with. In my experience, picnics take a little longer than butts, so you may want to add even a little more time. You'd be much better off getting done early and holding rather than waiting for your grub to get done.
There's a good chance your roast will stall at 160 degrees and hang out there for a few hours and then start climbing again. Some folks foil at 160. I'm not a foiler, but not for any reason other than I've just never done it that way. I'm confident many people who foil their butts still lead relatively normal & productive lives just the same as those who don't. Now whether smoking large hunks of meat is part of a "normal" and "productive" life is another story altogether...I myself have never been accused of being normal or productive, but I digress...
I pull the meat off around 190, but what you really, really need to do is pay attention to the texture - does the probe or an icepick or skewer slide in "like butta"? then it's ready to pull.
If you get done early, wrap that thing in a couple layers of heavy duty foil, wrap in a towel, then stick in a cooler. It can hang out there for up to 4 hours. It should sit al least an hour before you pull it apart. Since it's your first shot, don't plan on sleeping a lot on Thursday nite & don't open the smoker more than you absolutely have to or else you'll lose heat. Good luck!
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