There are already offline IDEs, that are way better than the online editor (with tools to rename variables, list all calls of a function, do step-by-step debugging, performance analysis, …). So why do we need an offline version of CG?
Running the tests offline is possible too (it’s called Unit Tests).
I haven’t tried it, but there seems to be a download script to get the testcases. Validators probably won’t happen, as these are to avoid hardcoding a solution.
You can even play most multiplayer games offline against yourself with the brutaltester.