You can go in and out of the gate area as often as you want, you'll just have to go back through the metal detector/wanding process each time. If there's no line, this isn't a big deal. However, if you have carryon luggage, and no one to watch it while you leave the secure area, that would add to the hassle.
The restrooms are just across the hall from the secured areas, so it's not like you have to hike a long way.
If you get there early enough for an A pass, I generally wouldn't be too concerned about lining up early to board. Unless your flight is one of the few flights that connect through MCI, you'll be one of the first 30-40 people on an empty plane, and good seats will be available. (If you poke around on the SWA website, you can figure out if your flight originates somewhere else. If it does, then being first among the "A"s might be worthwhile.)
SWA starts boarding about 20 minutes before the departure time, if I remember it right. I would try and be inside the secure area "for good" about 40 minutes before the departure time.