Hi,
To be honest, I just want to know if you have at least one method to verify that your hardware (custom carrier board) can output HDMI without any error.
Is updating dtb through flash able to give you this stable result?
If that is true, I think we don’t need to care about HDMI in your “update extlinux” case because there are plenty of other errors that we cannot guarantee whether it would affect the HDMI or not.
For the error after you update extlinux.conf, you could compare the full kernel command line between the working and non-working case and see what might be missing.