@zero: generally the rear head acts like a turbine when it functions like a thruster. Sucks in air from the front and pushes it out the back
But mostly, yes it's for ignition boost > : D
The dragon head can be reintegrated but it would put the Shenlong as a whole in a vulnerable position. That or if they try to perform it in highspeed then chances are that Rin would be seriously injured(I'm putting that lightly) or the IS would break because the locks that keep them in place take some time to properly secure(2-3 seconds?)
@eras: Ichika was just having those "my d**k is confused" moments. Some guys get that when they see a trap or a reverse trap. You know it's a guy but they looks too much like a girl that you wouldn't really mind. I'd like to interpret Ichika's blushes as more being ashamed than embarassed