I can answer your frist question. Because in their shariat, marrying a non-muslim/kaafir was allowed. In Islam, it is not so you have to break your nikah if your spouse turns away from islam.
by turning away you mean it literally, right? as in someone who denounces it, becomes an atheist or changes religion etc? as compared to someone who is just being lazy about namaz or roza...that is not grounds for divorce or is it? just wanted to clarify.