re: Understanding the classical model for linking: You can override an LIB with another LIB, and a LIB with an OBJ, but you can't override an OBJ

For the printf issue - printf OBJ should not include _fltused.  Yes if the app uses %f and does not use floating point the app will crash, but any app that uses floating formats will itself include _fltused so this symbol is not needed by printf