questions on smoking 8 butts in WSM

I have the same size WSM and the most I have done was a full shoulder, with the pic nic and butt separated. If memory serves, it was around 20 lbs. I don't think I could have done more than double that, because the top rack was just big enough. Considering that the middle rack is typically at a bit lower of a temp than the top (with heat rising), I wouldn't push the volume you try to cook on the bottom.

As far as time and temp, with pork shoulder being pretty forgving, I would try to hold steady around 300. Go with the minion, but for any long cook (like brisket) I usually have to light a fresh batch of coals in a chimney starter to keep it going. So get one of those if you don't have one already.

Honestly, with that amount, I would justgive the meat about 6-8 hours of smoke, then finish them covered in an oven. It will much easier for you to get the internal temp to the point that you can pull. You can still get a great result, and you won't risk the food not being ready when you need it. I've done shoulders well ahead of when I needed them, and just held them warm in a an ice chest wrapped in foil once they were done. When you're ready, the meat will still be piping and perfect for pulling.

ETA: This may go without saying, but insert you probe thermometer into the center of the largest cut you have. I would pull them all once that hits 190 since the smaller cuts should be higher and the carry over should easily take them to 200ish even if you rest them and serve immediately instead of wrapping and holding warm for later.

Yeah, you won't be able to avoid the bark softening up. Just something you have to give up if you're not doing the full cook on the smoker. That may be worth it, though, rather than risk the meat not being right when you need it for a group that big.

If you're just using the smoker, then I think rotating the racks is a must. Your guesses on the cooker temp sound right to me. I would definitely err on the side of a higher temp at first than rather lower. With all that meat, getting the internal temps to where you need it will be a challenge. I know 275 is as high as some will go with shoulders, but I think a good rule of thumb is to just keep it under 300. I've had no problem with dry out cooking shoulders that high. Just have your spray bottle or mop ready to keep the outsides moist.