1) Yes, you're not limited to using the pre-supplied ROG layouts, you can also build your own from scratch or use an existing LCD layout made by someone else.
2) Yes.
3) No, AIDA64 will refresh the whole screen periodically. AIDA64 is not watching whether the Registry values change, but simply read the actual values out periodically and use those values to render the LCD layout.
4) It depends on how Asus handles their own hardware using their own SDK (ROGAIOSDK.dll). But I'm sure they are sensible enough not to ruin the flash memory of the LCD device by unnecessarily updating it at each image transfer.
5) It is once again depends on how Asus handles concurrent access to their hardware via their own SDK (ROGAIOSDK.dll).
6) Last time we measured it, a full image update took 125 millisec on a 320x240 ROG AIO screen. Hence you can freely choose a frequent update rate without overwhelming the system, e.g. half a second would be perfect.
7) Administrator privileges are required by AIDA64 in order to poll sensor values like temperatures and voltages. It's not tied to the ROG AIO LCD module, but rather is a general requirement.