is there much of a difference between 'jaka wspaniała impreza!' and 'co za wspaniała impreza!'?
They seem very similar to me.
could one say "fajna impreza"?
you can although I am not sure if it translates to "wonderful" . More like "cool".
Can you say "Ale wspaniała impreza!" here?
Yes you can. It should be accepted now.
I have been taught that the adjective can follow the noun ("impreza wspaniały"). Is there a reason it is not accepted here?
It theoretically can, but it's relatively rare. It's more of a categorizing thing.
"impreza" is feminine, so it would be "impreza wspaniałA", but that sounds really strange in such an order anyway.
Take a look here: https://forum.duolingo.com/comment/21465404
Why jaka and not która?
"która" means "which".