I need to get the inertia tensor.

There is a function called get inertia tensor:

However as you can see, it returns a 3-vector.

The inertia tensor, however, is not a vector…it’s a 3x3 matrix, as explained here (and any other math website):

http://www.kwon3d.com/theory/moi/iten.html

What, then, is UE4 actually returning…and is there any way to actually get the full inertia tensor?

The full tensor is required in order to calculate the torque necessary to result in a given angular velocity.