You can check in via the mobile app. (I've done it even in meetings.

) You do not need to do anything beyond the screen where you see your boarding position. Don't need to save the QR code then, don't need to print anything.
It literally takes me 15 seconds or less. Probably less if you open the Southwest app before the start of your meeting.
Whenever you get at T-24 will be sufficient to avoid a middle seat. Even if it's B-something.