Christopher Reeves pulled off Clark Kent perfectly. I see no difference between Superman and Clark Kent in this film.
Good catch about discovering Krypton. Atleast on Smallville, they got it right.
For scientists to discover Krypton, his ship and the remnants of Krypton would have had to travel faster than the speed of light when it blew up, which it obviously did't since it took him 6 years to go and come back.
It would have been better if they had a better plot and just picked up where they left off as if he never left to begin with.