Sketchup assigns persistent IDs by giving each object a unique identifier that does not change when the object is modified.

SketchUp 2014 was bugged in this regard. The workaround was to pipe to a text file, I think. I think it was fixed in SketchUp 2015. I recall having written a patch script that overrode the Kernel.backquote method. (I have to have a look for it.)

Yea, even the Line tool has a lot of details. I've been replicating Rotate and Move in Vertex Tools. So much more than I originally though. Recently I started on a custom version of PushPull - it's giving me nightmares...