15lb bird in electric with a max of 275* chamber temp you definitely want to preheat...go with that. The danger-zone 40-140*/4hr guideline should be followed being it's not a fresh bird and you will be injecting it. Being you're stuffing the bird, your cooking time will be even longer, and getting through the danger-zone will be that much more difficult to achieve. If you take longer to make it through the danger-zone, does that mean you have to pitch the bird in the dumpster? No. It means you are now at an elevated risk of illness by consumption of that meat/poultry...lots of variables can come into play, so you don't want to
I would not inject again after starting the smoke as that will slow things down drastically by 1) opening the cabinet loosing heat and causing long recovery times (electrics can tend to recover slow), and, 2) injecting cool liquid into meat will cause then internal temperature to drop (the opposite of what you want once cooking has begun).
Do keep it in the fridge as much as possible until you start the smoke...don't delay after injection and rub application...handle quickly and return to the refer.
USDA inspected poultry should be cooked to a minimum of 165* I/T per a verified meat thermometer. I go to closer to 170* in the breasts and almost 175 in the dark meat due to pink to red areas near the bones at lower finished temps.
Provided your MES will maintain at or near 275* throughout the smoke (don't open the cabinet) it will take roughly 7-8 hours to reach ~172-175* I/T...depending on how much stuffing, that time could increase by a couple hours. That also will depend on whether or not you have a drip pan under the bird, as pans and other larger items will baffle the heat getting to anything above them, so keep that in mind. Your actual grate temps will vary top to bottom...most verticals run hotter on top, but any baffling will reduce that upper grate temp.
Let me know if there's anything I've stated that I failed to explain well enough...I'd be happy to go into further details if necessary, and post links to additional articles/threads and or announcements. Oh, speaking of, if you haven't read Chef Jimmy J's statement on the danger-zone in the heading of the Food Safety Forum, (clicky) that's a good place to start....quick read and very much to the point.