Why couldn't Fischer (Decaprio) just catch a flight to either Canada or Mexico and sneak in across the border to get back to his children?

Because that would still be illegal. He was looking for a legit means of getting in. And Saito offered it to him.

I know that they went into a deep sleep on the long plane trip, but how did they actually get into the mind of the son of CEO to implant the inception idea. I didnt quite understand that.

They all share the same dream. That is the basic idea of the movie. All the dream worlds were designed by Ariadne, but they were created by different individuals. The first one was Yusuf's world. The second one was Arthur's. And the third one was Eames'. And the fourth one was Limbo. Essentially when dreams are shared, so are the subconsciousness of all the people involved. That is how they got to Fischer.

I dont understand the scene where the van falls backwards into the river in slow motion. Can someone explain what that was all about.

Your mind processes information a lot faster than your body can keep up with. The general idea was that inside dreams, the worlds last longer because time moves faster there. The further layers you penetrate, the faster it becomes. 5 minutes in the real world is an hour in the first layer, and so on. So, the van falling slowly indicated that there was a deeper layer of dreams still in play.

Does the spinner spinning mean that he was still in the deep sleep, dream seeing as they each had to bring a personal item to let them know they were in the dream, and not back in reality?

It's an ambiguous ending. You are free to interpret it in many ways. One way is that Cobb's dreams were finally fulfilled. So, it didn't matter whether the totem was still spinning.