Horn, these are good questions. Paleoclimatic observation is key to answering your question and you put your finger right on it. The claims I make are based on paleoclimatic observations.  There are no GCM's ("models") involved in those observations. For instance, during the Pliocene period you mention (3mya) sea levels were 10m-30m higher than today

http://www.bitsofscience.org/2-degrees-climate-warming-pliocene-sea-level-rise-5353/

Think about what 30-90 feet higher sea levels would mean in all the coastal cities around the world. Think about the social and political chaos this would create.

Milankovitch cycles gave the initial push to ice ages and interglacials both--another good point--but most of the warming/cooling effect comes from the impact of GHG's, initially when warming oceans outgas, which gets the GHG feedback loop rolling. CO2 both lags AND leads natural warming events in the deep past.