Briskets in general become tender somewhere in the range of 195-210º because every piece of meat is different. If the heat applied is only 180º then ....? I would suggest a verified grate temp of at least 225º. Even at that temp it's going to take more than 12 hours. I'm assuming the 20lb packer, is the sticker weight. Once trimmed you should lose 3-4 lbs from removing excess fat. At 225º I would roughly figure at least an hour per pound (could be more) for a 16.5# brisket. Personally I run briskets at 275º and for Choice grade it's somewhere in the 45-47 min/lb and closer to 40 min/lb if Prime. To that add at least 1 hour (2 is better) for resting. Regardless, temp & time are only guidelines. Doneness is when it probes tender multiple places in the flat (feel is like probing a jar of peanut butter). BTW: if you pull when its tender and done, let it rest open on the counter for 10-15 min before wrapping and placing in a cooler. This will stop the cooking action, otherwise if straight into the cooler it will continue to cook, i.e. become over cooked.
Certainly it's your choice on wood to be used but remember mesquite is extremely strong. Some can stand it, others not so much. Considering you are smoking for a number of different mouths it might be prudent to smoke with hickory and put mesquite in the tube.
Seasoning: all I can offer is what I do and that is pure Texas Style. 50-50 salt and black pepper.