what is the logic between when grasshopper lets me set a point from grasshopper itself, getting stored internally into the GH definition, or when it wants me to pick a point referenced from rhino, it seems to switch selection method randomly on me.
if there's a point object in the Rhino document that will be the default option. If not, you'll be asked to pick a coordinate. You can switch between these modes via the command line prompt.
Also, if you've picked a point/coordinate on that parameter before it will use the same method.