What make you think that it's "switching" regulator? For me looks linear.
Is it ok to power via the VIN connection? I read that power is lost as heat through the connectors? It seems to most energy efficient way to power my project from the 12V source.
Powering via Vin you only by-passing a diode, reverse voltage protection. To by pass on-board regulator, apply external +5V to +5V input. So, to really save on power, you should search for another board, w/o regulator and serial/USB converter IC.