If one sets the raw import tool to use the camera's white balance the GUI elements for temperature etc. get disabled, i.e. the user cannot change anything. The bad thing about this is that they are not only disabled but also they are not filled with the values from the file. The user cannot check the values read from the RAW file. He can also not use those values as starting point and adjusting them. Hence it would be useful if the values read from the RAW file would be shown in the GUI, even if it is disabled. It might even make sense to allow the user to edit those values, i.e. not disable the GUI elements. Switching to method "camera" would simply fill in the values and the user can start from there. AFAIK ufraw does it like that. Reproducible: Always
What we can get from the file, and what is fed to libraw is not temperature and Green, but four RGB multipliers. We have code taken from ufraw that converts, for manual whitebalance, temperature and Green to these four multipliers. We would need add conversion code in the opposite direction, but also extend all this a bit more, to get on par with ufraw. (I do not have the necessary expertise for this.)