1. YUV to RGB Conversion : http://www.fourcc.org/fccyvrgb.php
The YUV range is [16-235] .vs. [0-255].
If the YUV range is [0-255] ?
The RGB range will be [0-255] or ?
If you cut the RGB in [0-255], some color will not be displayed.
There are many hardware solutions : Normalize, Level shifter, ...etc.
(0.7Vpp -> 1.0Vpp, 1.2Vpp, 1.5Vpp, ...)

2. Monitor's EEPROM (DDC EDID) will use the D-SUB Pin-9 5V or Monitor's 5V.
When the Monitor's AC Power line is unplug, the D-SUB Pin-9 5V is using for the EEPROM, so the PC can read the Monitor's DDC EDID without power-on the Monitor.
Problems : GND Level are not the same with PC, Monitor and TV-Box. It means that the protection circuit must be designed.
If the protection circuit is not designed, Monitor's EEPROM could be demaged, VGA could be demaged, TV-Box could be demaged.