Here are some answers in the same order:
1) You can use any user credential that has the vCenter privileges to access the Extension Manager. If you don't catch any error running that program then it means privileges were correct.
2) You can wait to copy your plugin .zip file to the correct location until the installer succeeds. The .zip file won't be accessed until the next user logs in.
3) You can check that the installation suceeded in multiple ways:
- Using the vCenter "mob" interface (go to https://<vcenter-ip/mob, login as admin, go to Content > ExtensionManager and see if your plugin extension id is in the Extension list)
- Login to vSphere Web Client: this will trigger the download and deployment of your plugin. You can also go to Administration > Plugin Management to check that your plugin shows up in the list.
If your plugin doesn't show up the first thing to check is the Virgo logs and look for your plugin id so see if any error happened.