Sorry, I don't quite follow it:
Do I need to execute the neroinstall.cmd RIGHT AFTER
I copy the file to NERO13820 dir, and then repack?
i supppose you mean "include" neroinstall.cmd and not "execute". Yes. include that file and repack. 7Zip must use neroinstall.cmd for execution.
run the neroinstall.cmd later in GUIRunOnce?
This will not work.
you have to repack including the neroinstall.cmd.
if you dont bother about the distribution size, you can run neroinstall.cmd (which you saved in the location where setupX.exe resides) directly.
if you wanted to repack and reduce the size ( to around 180mb ), use the link i gave before to know the procedure to make a 7zip silent executable. remember that neroinstall.cmd is the file which has to be called there.