How much change has there been to the shape of plate boundaries over geological time?
I am not sure if you are actually asking about Tectonic Plate boundaries, or the boundaries of continents, which are often described a puzzle pieces when teaching continental drift due to the way continents like South America and Africa fit together. I'm going to try and address both possibilities. 

Tectonic Plate boundaries are constantly changing. Many of the edges of tectonic boundaries are thin freshly-formed oceanic crust which isn't going to put up much of a fight. You know about subduction so you understand that oceanic crust is constantly 'recycled'. 

The present day amount of continental crust on earth hasn't existed for all of earth's history. Current thinking is that it has been been accumulated overtime (theories differ as to how quickly), through a number of processes which have built up the present level of continental crust that is more buoyant that the mantel and oceanic crust and thus resistant to subduction. As a result the size and shape of the continents have been slowly changing and growing over earth's history as they gain new bits, join and split. 


  For example, what would the 'jigsaw pattern' of plates at the time of Pangea have looked like?


this video shows continents moving around, growing and joining and how they change over time, at least for the last 1.5 billion years of earth's 4.5 billion history. Pangea was around from 335Ma to 175Ma, you can see it form and break up in the video.

https://www.youtube.com/watch?v=IlnwyAbczog


  This probably relates to the potential for new plate boundaries to be
  formed...and existing ones to close up. Is this possible?


In this video blue and red lines show active tectonic plate boundaries.
you can see that they change. They move around, old ones stop and new ones start. They even change from divergent to convergent or consume each other.


  At plate margins, does the pressure of compression or expansion actually change the spatial position of the margin?


Absolutely. at the end of the video watch India slam into Asia and create the Himalayas as a result.

I'm not an education specialist, but i have seen videos like these be used to explain the processes to kids.