The load balancer only works within one region. If you want to balance traffic between different regions you will need to look at latency based routing in Route 53. Keep in mind that availability zone and region have different meanings within EC2
MongoDB replica set is a flexible master/slave configuration. If the primary instance fails, a secondary, based on configured priority can automatically become primary. Network within a region is fast, you will have some latency if you use multiple regions.
EC2 instance can access an s3 bucket in any region, you wont pay for outgoing bandwidth if both are in the same region.