The system return code ($?) is, with few exceptions, the status code returned by the last command executed on the command line - in this case, this is the tee command. Thus, since it [the command] is a pipe, the RC from the first command on the CLI is unobtainable using the command in the form shown because the first command completes moments prior to the tee command. to address your questions...

1. You give no clues as to the C:\\IA71_Enterprise\\build.exe, but assuming a similarity to make(1), you could introduce a dummy target (as the last dependency of the all target) to create a tmp file iff the build completed succesfully. Alternatively, you could parse $buildLog (using perl of course:-) for error condition strings or... You could utilise the suggestions of one of the other respondants.
2. As far as redirecting STDERR is concerned, I didn't think DOS recognised either STDERR or filehandle 2 ... so, once again, I would refer you to earlier posts regarding the use of perl to do the job properly.