in a setup like this, since the midrange is low in the door, you would want your tweeter in the same and on the same sound plane. if the door speaker was a midbass driver then you could move the tweeter and midrange.
what's wrong with a cap?? nothing!! i've you're not using a lot of power all the time, a cap is all you need. now...if you're running high volume all day long, then yeah, ditch the cap and get a storage cell (extra battery as some put it)
as far as rears, i don't run them, most SQ builds don't run them, most concerts don't have them. typically if a customer wants rears, i use a coax, and turn them way the heck down. the main reason most new cars have component style speakers in the back, is because they cater to families and business class where there is almost always more than 2 people in the car.