if you ont plan on doing anymore then the 4 guage should be fine, but if ever plan on adding more you can just go ahead and put 0 guage.

not sure what your backseat looks like but you could mount the amp the the back deck. if you put the amp b/w the subs that is going to be a big box and you may have touble getting it in the trunk, you might have to build it in the trunk. yeah you could put the amp vertical but then the wording on the amp might look odd.

not familiar with many cars engine bays but i know my alt doesnt have a ground strap. all you need to do is match your ground wire and power wire. so if you use 4 guage from battery to amp, the amp should have 4 guage ground as well the battery.u dont have to replace the one you have just add a 4 guage wire from the battery to a part of the frame.

if you plan on doing all this to the backseat whats going on in the trunk??