My assumption is that the asymmetry you note is a function of albedo feedback off the ice sheets and the fact that it's a lot easier to destroy an ice sheet by having it all slide out to sea than it is to build one (which has to be done one year's snowfall at a time).

On past interglacials - if you go back 400k or so, there was a fairly broad one.  There is a debate on at the moment started by Ruddiman as to whether the holocene would have been shorter if it wasn't for low level forcings due to human agricultural development (ie long before the industrial era).  He summarizes his arguments in a very nice book called "Plows, plagues, and petroleum".  He thinks we would have already have been starting the long slow descent into the next ice age if it wasn't for the invention of agriculture.  However, there's certainly no consensus around that idea: others think it's just the way the particular combination of orbital factors line up.  There was a good discussion of this recently over at RealClimate.