Has an entire plate ever subducted?
Excellent question!  Indeed, entire plates have subducted before.

One example is the Intermontane Plate (Wikipedia).  This plate sat west of North America around 195 million years ago.  The plate contained a chain of volcanic islands on its western edge (known as the Intermontane Islands).  As the plate subducted under the North American plate, that island chain stood too tall and merged with the North American plate.

An illustration of the plate:

Source: Black Tusk, Wikimedia Commons 

The entire Intermontane Plate was subducted under the North American plate.  However, we are able to see the remnants of the plate in the volcanic islands it left behind.

Once this Intermontane Plate was subducted, the Insular Plate became the new subduction zone.  

This entire process was actually repeated with the Insular Plate, subducting the entire plate, leaving behind yet another chain of volcanic islands that fused to the western coast of the North American Plate.

The Burke Museum has an excellent article that describes the subduction of both the Intermontane Plate and the Insular Plate.  Well worth the read.