Why does hyperledger-fabric need more than one orderer?

You will find full answers to your questions in the Fabric Docs, but quick answers below:

1. In Hyperledger Fabric - (you have the process roughly correct in your description) a single orderer is a single point of failure, and a single unit of capacity - a multi node orderering service can be more resilient and more performant.
2. Organisations would add multiple peer nodes for resiliency and capacity
3. Yes, you could connect using the Node SDK or GoLang SDK. You would have (create) a connection profile that describes your Fabric Network of Nodes which would contain amongst other things the URLs needed to connect to the nodes.