Update: I was finally able to upload new code to the board by repeatedly clicking the Upload button, and ignoring the /dev/ttyACM0 errors. On the 4th or 5th click it worked, and my Arduino is no longer stuck in an infinite Serial.print loop, which makes it behave much better.
In summary, Arduinos that have programs that stream out serial data are very difficult to program / work with, even outside of the IDE, as evidenced by minicom's issues.