There are 12 "main" functions also(!); haven't encountered anything like it since I use IDEs.
Then clearly, obviously, the "project" was not meant to be compiled and linked the way you are trying to do it. There are at least 12 separate sub-projects here (12 executables plus possibly dynamic or static libraries), each of which must be compiled and linked as a separate unit.

It is possible that the main() functions are part of a unit testing framework, where when a certain pre-processor #define is present (or not), it compiles and links an executable that tests that part of the project.

In any case, if the "giant project" was not designed for compiling with QtCreator, then why are you trying to do that? If it uses Gnu Make or some other build tool, use that. But adding a qmake hack to force 12 separate entrees to be cooked in the same pot is not the way to do it.