Most of this has been covered in greater detail within the forums. If you search using a few nouns from each question I imagine the answers will be better and more in-depth than mine below but you asked so here goes.
1) I would place the AMP either under the driver's seat or in the boot.
Use new speaker cables, follow the path of the existing speaker wiring as much as possible. I wouldn't use the existing wiring, it's likely to be to thin.
2) This is easier if your headunit has an aux in, just connect the soundcard out to the aux in. If not, you could use either an audio line-level switch box or a mixer on the line levels from the headunit and the soundcard.
4)Yes if the total current is less than the power cable is rated at.
I'm afraid that the easiest solution is often not the best.
Good luck, and check other opinions on the above, it may give you more ideas.