--And if so, is the "mraa" library the most efficient way to program the uC ?
Yes MRAA and UPM on GitHub most example code in C C++ and other.
--Together with a sketch, which uses f.e. shared memory with a Linux process (supported by the Intel "Arduino" IDE) ?
Arduino IDE includes the cross compiler (exactly compiler on Edison Linux System)
Debugging and programming is excelent with Intel XDA. Arduino IDE can only debugging with Serial Port.
--Is it possible to compile a sketch on command line (without IDE) on Edison ?
Yes write you code and compile it with make or cmake on commandline.All header and library are included on system. (see example file on GitHub)