Hi Paolo,
1) Yes
2) Yes
3) Yes
4) All major languages are available: C/C++, Python, Perl, Ruby, Fortran, VALA, LUA, BASIC. The Python GPIO modul is available in the repository.
5) piCore offers the same graphical envionment as TC, WBAR + FLWM_TOPSIDE + FLTK, this is piCore-X. LXDE is also available. JWM is in progress.