make sure when you do open up that plastic cable management grommet and remove all the electrical tape, there will be 2 thick cables wrapped in electrical tape. The one to open up is the one where there is a thick black and thin black cable sticking out of them. And pretty much look at the pictures for reference. When you remove the tape, there will be that grounding cable that connects to 3 or 4 other silver grounding cables. Itll be connected by that metal clip. And you start adding solder onto that metal clip. So it makes good contact.
My guess as to why these misfires happen during idle is because when the engine is running, the vibration from the engine will make the ground connection shake also. Making the ground connection lose its ground momentarily. So thats probably why it misfires.
So when youre all done, just use electrical tape and tape it like how you found it.
If that doesnt solve your problem, then i guess back to subaru it goes. But im sure it will fix it.