Is there a chance for paper wallet script to generate an existing address that someone else uses?
Depends on how much you'll hem and haw over what constitutes "a chance". Realistically, assuming you used a strong random number generator, it will never happen in the lifetime of the universe.
Also, can someone hack the private key for a bitcoin address and sweep the entire balance, maybe FBI with super computers, maybe Asic private key decrypter?
If you reuse the paper wallet after cashing it in once, it would require quantum supercomputing or some heretofore unknown fatal vulnerability in the math of ECDSA. If you don't reuse public keys, it would require that
plus some heretofore unknown fatal vulnerability in the math of RIPEMD-160. Without those? It returns to the answer to your first question: realistically, assuming you used a strong RNG, they will not succeed in the lifetime of the universe.
(Note: these answers also apply to single addresses.)
