Yes, if either HDD/DVD are dead, you should get an error message at boot.
If no video or sound is heard when you try to boot, it means either a bad video cable connection or the motherboard itself failing to boot.
When the Xbox fail to boot, can you switch it off by pressing the power button or does it stay on unless you unplug the power cable?
There is something else that happened to me once too, it was the DVD drive making the Xbox not booting.
When I unplug the drive (bother power and ata cables) it boots correctly but showed an error.
You can try that even if I don't think it's the same problem has you have.
You mentioned that the gpu fan is getting some power but not turning? So it might be that there is something wrong with it, or the motherboard fail to boot, and doesn't provide enough power for the fan to turn.
Either way it does have something to do with the motherboard.
On a side note, I wonder if it's possible to mess with the Xbox bios from a softmod?
Indeed checking with a normal AV cable might be a good idea, just in case the RGB cable is not working somehow.
Good luck with your Xbox fixing, hope you can make it work
