Hey,
sorry for late response. I did some more tests with the patch I proposed and I found out that the problem is not trivial. I'm not even sure if it can be fixed. The problem is that when you call
subProcessInstance.setSuperProcessExecution(null);
you break the link between child and parent process. And this link is necessary in SubProcessActivity to extract return variables.
Also when you suspend your subprocess you don't delete it from the database which is a big problem of version 4.3 in general (I think it has been fixed in 4.4).
Currently I'm testing a solution with subprocess deletion in SubProcessActivity but it still creates some problems.
I'll keep you informed.
I would be cool to have an opinion from someone from the development team on that. Anyone?