@Tobias_Fleischer and @David_Tschumperle
Thank you both for your help.
I think I’m through with my current set of issues 
It’s possible to set the global variable _path-rc at runtime. So, no problems on that part any more.
For the inplace issue, it’s been solved by using version 2.0.
For the last issue, it’s not the delete_external that is crashing, but an error in the code.
I had copied this line from the uselibc example:
gmic_interface_image images[1];
When there is more than one image, the stack cleanup at the end of the method crashes.
Changing to
gmic_interface_image images[100];
solves the issue at the cost of a little memory overhead.