The lightning bolt indeed means undervoltage. This can be due to an improper power supply or USB cable.
I'd recomment using the official RPi3B+ power supply, this is known to be working fine.
As undervoltage can cause all sorts of odd issues it's best to fix this one first - the CEC issue could be a consequence of undervoltage situations.
BTW: you can also check with `vcgencmd get_throttled` if any undervoltage (or overtemperature situation) occurred since bootup - you should get `0x0` printed there,
so long,
Hias