Yup, the earth spinning doesn't make sense in Superman 1 either. Technically, as he is pulling the now-alive Lois out of her car, the nuclear missile is still sailing around, which he never re-saves--that was the whole point of the original dillema, he could save the world from the missile or he could chose to save one human whom he loved, and of course he had to go with the human race as a whole. The reason, however, that audiences accepted it is that the matter of Lois dying is such a huge emotional shocker and Reeves performance so harrowing that you truely do believe that the man's willpower is capable of anything at that state. Superman II by comparison is an intellectual one, where everything is already fixed, he sits down, thinks about things, and then decides that reversing time would be a good idea.

The problem with the Superman II ending is this: its not the real intended ending. Lets say that it is ending v0.5. The original plan for the two films were as follows: Superman 1 would not have any of Lois being killed by the earthquake, the climax would be Superman racing to save the missile. He throws it into space, where it detonates and releases the Phantom Zone prisoners, who yell "freedom!" before advancing on earth--cut to title: "To be continued next year in Superman II!" Bum Bum BUUUUUUM! Basically it was the prologue of the Donner Cut of Superman II. Then Superman II would end as the Donner Cut did, with Superman reversing time. But after they abandoned Superman II 3/4 through shooting and focused only on Superman 1, they realised that the reversing-time ending would better suit Superman 1. Thus, the whole structure of the ending of part 1 and the beginning and end of part 2 was totally re-written. But then Donner quit/was fired before they could come back to complete Superman II. So the ending was never completed. What we have on Superman II The Richard Donner Cut is the original ending, but technically Superman 1 would have to be re-edited to be in its original configuration. I don't know how Donner would have solved the memory thing--the "magic kiss" of Lester's is not so bad, but probably Donner would just have Lois with the knowledge that Clark is Superman. But anyway, Lester's ending is not the true ending either, and in wanting to use as much Donner material as possible, Donner's original ending was kept intact, even if it raises some plot questions and would have been thrown away anyway.