Is it correct to say that LAB is the "native color space" for InDesign? That colors specified in RGB that need to be translated into CMYK will go through LAB along the way, and vice versa?
No, LAB is not the native color space of InDesign. InDesign does not have a native color space.
However, all color conversions between color spaces is done via ACE, the Adobe Color Engine, a component of Adobe's Core Technology, using ICC color management techniques. All conversions between color spaces are made first to a connection color space and then to the target color space. LAB is pretty much what that connection color space is.