How to modify registry keys for default user

[rsbrux]

Before creating additional logins on my newly installed Windows 8.1 Pro x64, I would like to make some changes in the registry to apply to all newly created users. In Windows 7 I did this by making a custom unatended installation USB in which I loaded the default user's hive from %SystemDrive%\Users\Default\NTUSER.DAT and modified it using REG before finalizing the installation. The Windows 8.1 installation is a one-time installation, so it isn't worth the trouble of creating custom installation media. Furthermore, I have read (e.g. here) that one can do this post-installation. IAC, my basic problem, not knowing which file to load the default user's registry hive from, applies: I haven't found out where the default user's hive resides on my system. Documentation for Windows 10 points to %SystemRoot%\System32\Config\.
My installation has both %SystemRoot%\System32\Config\DEFAULT and %SystemDrive%\Users\Default\NTUSER.DAT.
Which is the default user's hive?
I am inclined to think it is the former, as it is large and has a more recent modification timestamp.
I don't even find a way to edit the default user's registry in regedit. I only see HKEY_USERS\.DEFAULT, which is not the same thing.
How can I edit the default user's registry?
My preferred method would be a script file using REG commands, e.g.

REG SAVE <registry path to default user profile> <path to backup file for default user's hive>
REG LOAD HKU\DefaultUser <path to default user's hive file>
REG ADD ...
.
.
.
REG UNLOAD HKU\DefaultUser

but as you can see, I am missing a few of the required parameters }:-¢
Any suggestions?

P.S. If this belongs in "Customizations", feel free to move it.

 

[rsbrux]

It appears that:
1. The default user's hive resides at %SystemDrive%\Users\Default\NTUSER.DAT.
2. The hive file %SystemRoot%\System32\Config\DEFAULT corresponds to HKEY_USERS\.DEFAULT.
3. The default user's hive is not usually loaded, which is why it doesn't appear in regedit (unless you load it yourself, as I propose doing with REG LOAD in my example above).
I am proceeding on this basis and will report back on the results.