I managed to build GTK+ 2.24.30 for Win32 (and all of its dependencies) overnight so I recompiled and tested the DC2N4-LC client.
If I use Cairo there is still an issue with the DC2N4-LC client, surely kicking in while processing the drawing area widget’s expose event: if part of it leaves the screen and comes back in then the application crashes. However, if I don’t use Cairo then the DC2N4-LC client does NOT crash and shows a steady memory usage during its whole lifespan: there are no memory leaks in the latest GTK+ 2 source for Windows!
What I need to do now is to package the runtime for other users and rebuild my GTK+2 applications using the SDK I built. I might also update the MinGW toolchain I use to compile my apps to the one I used to compile GTK+ 2.24.30 as it supports building 64 bit binaries.