The second amplifier is going to be on the trunk floor flushed with the current enclosures, so I will loose about 2" max.

The factory amp is not all that large and it is unlikely you will find an aftermarket unit that will *** behind the plate, HOWEVER, the foam that separates that plate from the carpet, might be an opportunity to build a small - fan cooled enclosure for a small sized amp - caution though this area of the car is noisy and could introduce some issues.

VQ is much larger than stock subs so I would venture to say no, unless you build an enclosure back there. I have yet to take the back wall completely apart, but I would believe by the time you laid down some fiberglass the woofer might restrict the seatback from going all the way back.

Not much in the 2 ohm department on the aftermarket, Eclipse has some at one time, JBL will have some in their new GTO lineup this spring.