Welcome to Flyertalk, OP.

Don't stress about being IDBd from the flight - as you remembered, it is extremely rare. In any event, the absence of a seat assignment does not make you more likely to be bumped. AA's IDB priority factors include last to check in (always check in early), elite status and other factors. Not having a seat assignment doesn't make you more vulnerable.

The plane is probably half empty but many of the open seats won't appear to non-elites when they look at the seat map.

If it's a long flight and you're tall (long legs), then pony up for MCE if you want to be certain you'll get it. Other wise, check in online well before your flight and play the seat assignment lottery. Maybe you'll get lucky and get an MCE seat.