You're halfway right ^. The heat element is used for start, and some afterglow when its cold. In most commonrail engines the glow plugs are also used to measure the pressure in the cylinder:
1 Plug
2 Circuit board with electronics
3 Glow plug body
4 Glow plug heating rod
5 High voltage connection
6 Measuring diaphragm
7 Gasket
There are a few articles on using the glow plugs to measure ionic conductivity of the combustion process, but it seems the pressure sensing is what the OEMs settled on.
An induction heater might loosen the plug if you can get it in there - thats worth a try if you can borrow one
