An Egyptian soldier crossing the Red Seas with the Hebrew would not be a strange sight. If you pay attention during the actual Exodus from Egypt, you can see some Egyptian soldiers throwing down their weapons and join the Hebrews (that is mentioned in the Bible if I am not mistaken). Its no that the soldiers you saw were chasing the Hebrews and not killed, they were with the Hebrews and crossed the sea with them.
Yes. This is exactly why the Old Testament has laws about how the Jews were to treat forigners (sp?) living among them. Many of the Egyptions packed up and went with them (smart move after seeing the God of Moses defeat thier own gods).