Well maybe not the big difference in magnification..it would be the same, but in contrast yes that would make a difference.

The rear port is used for accesories like 45 degrees diagonal (this is more for using it like spotting scope, not actually astronomical), camera adapters and so on.

The Zoom eyepice is a good option for the ETX-80, so yes thats for not changing the eyepices, the only minor thing is that more glass on a zoom or eyepice means less light trnasmission, but fot the ETX-80 i think the zoom eyepice is the way to go.

A really good zoom eyepices would be Baader zoom (8mm to 24mm)...i think Orion tooh ave a preimum Zoom very good, lower end ones would be like Celestron zoom and meade zoom.