You’re attempting to start it as a regular user. You need root privileges for that.
any such command will prompt for authentication if needed, no?
that is my experience
and the attempt did end up in, or would have ended up in:
No, not systemctl — at least, not in my experience. systemctl can also be used for starting user-level services, so it needs to be told explicitly what it must do, and connecting a network device is a privileged operation.
will it not prompt for authorization when privileged actions are required ?
Not in my experience, no.
In this particular case, it would just attempt to start NetworkManager with user privileges, and so the failure does not come from systemctl itself but from NetworkManager, which is told to do something for which it does not have permission.
No, it will not. This is not how systemctl start works. The unit is always startet as the user listed in the service file, if not user is specified, the unit is started as root.
If the option --user is used, systemctl will interact with service manager of the calling user, however system units like NetworkManager.service can not be start by the service manager of the calling user. User units need to be put in specific folders.
Also, systemctl will ask with polkit for elevated privileges if need. It works like pamac. Of course polkit needs to be working, otherwise it will not work.
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mkay - perhaps
I know I can start/stop/anything NetworkManager
after having been prompted for the proper credentials
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Systemctl offtopic aside, the file command reported wrong file type. So we know for sure it was corrupted. How did it happen we do not know. Might be the filesystem (just check it from a working live iso), or the drive which is new but one never knows (systemctl, smart status), or the transferred file, the iso.
So, wait for the new usb, install ventoy on it, download a fresh iso, verify the checksum on the usb iso, boot from it and fsck the rootfilesystem of the installation withouth mounting. Or reinstall and then check to avoid the possibility of other corrupt files from the old defect iso, if that was the cause. And finally smartctl the drive.
That way you will be 100 sure and will have guaranteed clean system. And if that happens again it is probably a defect disk, even if new.
that is the way better option 
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… with matching SHA256sum – how probable is that?
No. I know how to use su -
Long story short:
I erased the USB stick, wrote the ISO once more (Manjaro Gnome / Nautilus / Disk Image Writer), re–installed the computer – it failed at boot time / GDM. I then took a new stick from a friend, installed the machine for the third time: no problems occured, system up and running flawlessly for more than 12 hrs now¹. — So I still blame that old stick to be faulty → trashbin.
¹ now starting a copy & check loop to test the new NVMe
Thanks again everyone. 
and then you could do the sha256sum check on what ended up on the stick
not possible if you use some other disk image writer software.
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It is worth noting however, that ventoy will not be able to boot every iso out there. Depending on how the iso is made. At least 2 live recovery cd from big linux unfriendly antivirus peoviders are that poorly made. Just not easily chainloading, the path to the kernel is hardcoded and looked on the wrong loop device.
But all major linux distros should be ok.
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