When installing the agent manually, it prompts for credentials. I want to clarify what these are used for.
1. Do I need to provide the agent with credentials that have local admin rights (in order for it to collect WMI information since it runs as a service)?
No. The credentials you are asked to provide are your Orion credentials. These credentials are used to register the Orion agent with the Orion server or additional poller.
2. If the Orion Web Console allows me to download the MSI with MST for a silent install via group policy, then does that mean I have to create a second policy to tell the resulting Agent service to run under an account with local admin rights? (the equivalent of manually entering it in the screen shot below)
The MST file is bound to a specific polling engine based upon the options selected, so there is no requirement to provide credentials of any type for the agent to register with the Orion server when mass deployed using the associated MST file.