Dude you are making this way more complicated than it is.
That adapter from eBay is pretty much what you need and not bad price.
You really don't have to touch the back of the head unit at all.
You can tap into the wires from the factory amp behind the back seat.
As for 12v power and ground you can use use a bit of extra wire from the new amp to the lc2. Simple daisy chain.
This vid is a bit hard to see but just pause it a few times. That's how quick and easy it is.