Can I retrieve an individual backlog item or other object from the API?
No, the only way to retrieve data from the API is using the getData method. You should parse the data from getData into a suitable data structure at your end, and then retrieve the objects that you need from that data structure.
This requires a bit more work at your end, but on the other hand, you always get a consistent snapshot of all of your data, at that exact moment in time, regardless of whether somebody else is modifying the data at the same time.
Make sure to use the includeProperties parameter in getData to get all the data that you need included in the result.