There is also the offline registry editor in the boot env (http://pogostick.net/~pnh/ntpasswd/ ) so it should be possible if it is needed. (is it?)
Yes. See usetup sources (base/setup/usetup).
For automated installation it would be helpful to have a possibility to place the answer file directly into the InstallDIR (e.g. ReactOS\ or ReactOS\system32). What do you think?
Then you should modify the installer (syssetup).