Maybe you're thinking this is more complicated than it needs to be? I only use ONE large 15" mono sub in each of my two cars. I'm now powering it with a 450W amp in my Volvo wagon which is more than enough. With an aftermarket sub amp, it doesn't really matter where you decide to tap into your speaker wiring but I'd just tap into the two closest to the trunk (or amp location, easiest way). A mono sub amp takes both L and R inputs and combines the output to one single speaker output. One good powered sub is more than enough IMO, one doesn't really need 2 subs but many people do that too. The surround sound, etc doesn't matter at all when adding a sub amp. Don't compare anything aftermarket to how Volvo does it in the C70 convertable as it won't matter if going aftermarket.
Rather than keep asking people here, why not discuss it with people at stereo install places to get their opinions? It'll all end up depending on how much bass you want and how much money you want to spend.