From what the seller told me, the unit was working previously, so I don’t think is related to a solder bridge.
Also, would it be possible to have a cold solder joint occur after the initial soldering was carried out?
The only console I had to hand was my original Xbox with component cables, so I tried them in AV2 and the unit appeared to recognise the device (the LCD changed) and it did eventually display the picture from the Xbox on the screen, however it looked awful as the test pattern does.
The seller also included a DVI to HDMI adapter, but I used one I already have as well which I know to be working, so at least we can rule out a simple cable issue.
Is there a simple way to test the three things you mentioned without switching out the components? So I only have to potentially replace the one thing that is at fault.