Long-tailed pair - gain and bandwidth
The bandwith of an input stage as differential pair is strongly dependent from:
 1. DEVICE: each transistor has a cutoff frequency fT=gm/2/pi/Cpi/Cmu

 2. CONFIGURATION: simple differential stage has BW dependent also from impedance of current mirror you use  

Check this book for more details https://www.amazon.com/Analog-Circuit-Design-Discrete-Integrated/dp/0078028191 

For question 2 the BJT is a Vbe multiplier in my opinion is used to keep the JFET in the linear zone, becuase if it saturates then the BW collapse (you are not anymore in the linear zone)

For question 3, check the book the gain is frequancy dependend in low freq is -gm*(Rc//ro)

For question 4, I don't see where R14, R16, R12, R13 are. the output in small signal model is the same of common emitter stage normally quite large