But what is so special about the win32_ExitProcess() function, the 
changes apply only for the actual ExitProcess() call (not the duration 
of the rest of the forked job.

That looks like a forced fix to a specific problem, not a generic 
buildbot-friendly feature.

SetErrorMode(), btw, is inherited by child processes:
"Each process has an associated error mode that indicates to the system 
how the application is going to respond to serious errors. A child 
process inherits the error mode of its parent process. To retrieve the 
process error mode, use the GetErrorMode function."

For _CrtSetReportMode, don't we want to channel the output to stderr, 
rather than the debugger output?  No one has access to the debugger 
output of a child process normally.
You can get the handle of stdout with GetStdHandle(STD_ERROR_HANDLE)

Also, let's not forget, that parameter validation will throw up 
dialogue boxes in a release mode too() (in DEBUG an assert is thrown in 
to the mix as well, which is what you are silencing)

If we are doing this for the benefit of buildbot, then why don't we do 
it in python.exe?  We could add an --unattended flag or something 
similar which would silence or redirect any interactive notifications 
we can think of.