check your RGB/SCART cable is correctly wired with no extra or incorrect connections using a mutimeter in resistance/continuity mode. Without removing the connector covers, check all pins and shields on each connector to all pins/shields on both connectors.
Also, check the connection resistance and double check that the pin numbers are correct. Especially if you're using a DB25 25 pin connector that has been modified (cut down) to be a DB23 23 pin connector.
For the harder part of checking for A1200 damage, you'll need some Amiga 1200 schematics:
http://www.amigawiki.de/doku.php?id=...ice:schematics
https://archive.org/details/A1200_Sy...1992_Commodore
http://www.jammarcade.net/computer-schematics/
and Amiga 1200 hardware info from
http://aminet.net/search?query=A1200+hardware (in amiga iff format!!)
There are also many Youtube videos on fixing Amiga 1200
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The A1200 schematics should match the A1200 board version.
http://www.amigapcb.org/ only has the Amiga 1200 revision 2b Motherboard at present.
Suspect that the pin 22 to +12v line connectivity issue was because the connection at the +12v end was bad. Try from video port pin 22 to each side of nearby SMD resistor E264 instead; one should give near 0 ohms resistance and the other about 200 ohms resistance. Then try pin 22 to the positive pin of the large 47 micro-farad SMD electrolytic capacitor C821 near the Amiga 1200's power input socket; we expect about 200 ohms resistance.
Doubt that shorting any of the VCC (+5v), +12v, or -12v pins on the Video port to the 0v pin or shield on the same port would actually damage the A1200 Motherboard traces as each is currrent limited by an in-series 200 ohm resistor to a maximum short to 0v current of 25mA, 60mA, and 60mA respectively.
Given that you're VGA cable using the same port is giving a good signal then it is probably an incorrectly wired RGB/SCART cable causing the wrong voltage levels (higher than expected) to appear on the RGB (or composite video) pins of the SCART plug.