And yes I totally agree with you about needing to compile your own Glade 3.8.5 or 3.8.6
It's even easier to compile than Geany.
but until support for GTK2 is removed from Geany, the Glade version can't be advanced to one thats available in repositories
That's not true, if we advance the Glade version, it will still be stuck at a different version, which is unlikely to be the same version as found in <your distro> repositories. It has nothing to do with GTK+ version but rather that the output from Glade is unstable, even between minor versions.