RB3011 can be power by DC jack and via ether1 24vdc PoE. Can it be powered both DC jack and PoE at the same time? Advantages? Firmware update issues?

I also read a post that claimed using both inputs at the same time solved the RB3011 "flapping" problem.

Yes both can be used, and they will also work as active failover if one of the feeds fails. I do the same with old RB1100AHx2's (As they only had single AC) and 3011's in production