[old_tv_nut]

1) yes, you can use an external 12 volt supply.
1b) post a schematic of your circuit - it may be drawing more current than necessary depending on the resistor/pot values you used.

2) When you say you adjusted the amplifier to match the internal signal, is that a DC voltage reading of 7.5 volts? You need also to match the peak-to-peak voltage, and your waveforms look like the injected signal is much smaller.

3) the internal signal has much bigger sync than the injected one - your amplifier may be clipping sync. It would be good to buy a DVD with test signals like color bars that have known video levels instead of looking at program material where the video level varies scene-to-scene.