I think you should give the specification back to a functional consultant and ask them to clarify this. BAPI does exactly what its name says - checks availability. It does not mean material is in stock, it just means that material can be available on specific date. Also this BAPI is a bit slow, I'm sure you can find the stock figures faster in the inventory tables. But how exactly - that's the question back to MM consultant.
There is no FM for "backlog" and there is actually no single definition (or rather SAP definition is not always the same as customer's). Heck, in our company even Sales and Accounting can't agree on what "backlog" is. I'm guessing you'd need to look for certain sales orders, but specific criteria needs to be clarified by SD consultant who was supposed to capture the business requirements and ask what exactly "backlog" means.