Can't believe you kept it all that time without trying it!
The manual says to hold down either shift key, so I don't know what the problem is, maybe test if both shift keys are working ?
As for the "Can't open video screen", I think it might mean that your current screen isn't compatible. I remember being able to deselect maximum and getting it to open on a window, but that message would appear on some screens.
BTW, if you have a VCR or DVD player with SVHS output you can connect it to the Paloma's SVHS input. If it has a tuner better you won't need to use the aerial then.
I can't use mine now as I tried to move it to an A4000D and it didn't work there, and I've meanwhile took off the HD on the 4000T to upgrade it but really haven't had the time to finish it yet...