I was able to answer my first question based on this discussion:
Typing “systemctl mask serial-getty@ttyS0.service” returned “Failed to mask unit: The name org.freedesktop.PolicyKit1 was not provided by any .service files”.
After adding sudo: “sudo systemctl mask serial-getty@ttyS0.service” and rebooting, the service “serial-getty@ttyS0.service” disappeared from “systemctl --full”.