Bob, if you plan to smoke at 140, please add Instacure #1! A teaspoon, in your brine, will be fine. Low-temp jerky must be cured, or you are asking for trouble! Personally, I believe ALL jerky must be cured, no matter how long it lasts! The power of curing salt, in combating botulism, cannot be overlooked!
No need to rotate the racks. And, hopefully your disposable grill mats will provide smoke to all sides (mesh?). I haven't seen these. I use Q-Matz, from Amazen, and they're great. At low temp, though, no need to rotate.