If you run Rainmeter with a parameter, and the parameter is not a !bang, it sees it (I think leftover from the ancient days when Rainmeter was a "plugin" for LiteStep) as a "load" command and assumes you want to use an alternative Rainmeter.ini. With the quotes around the !bang, it is not recognized as such, and so you get the behavior you describe.
You are getting the error due to UAC stopping you from creating a folder in C:\Program Files\Rainmeter I suspect.
If you don't have UAC on, and run "C:\Program Files\Rainmeter\Rainmeter.exe" "!RainmeterQuit" from the command line, you get another instance of Rainmeter running, and a new folder in C:\Program Files\Rainmeter called \!RainmeterQuit. It will have a default Rainmeter.ini in it.
Just be sure the !bang itself is not in quotes. Parms to the !bang that have spaces can / should have quotes around them.