Pivoting in response to installing a NC21 beta package:
Would the NC 21 beta co-exist with mainline NC21 when released (not optimal), or
Would the NC21 mainline package (when released) update the NC21 beta install, so I only have one NC install? (preferred; as opposed to having NC21 and NC21 beta both installed)
Would the NC21 mainline package (when released) update the NC21 beta install, so I only have one NC install? (preferred; as opposed to having NC21 and NC21 beta both installed)
Yes
Of course you could install NC21 as a seperate instance on the same server, but if you perform an upgrade, either in the web interface or via command line, the existing files are overwritten. So there is always only one version installed on your server. In order to upgrade to the beta version of NC21 you have to switch the Release Channel" to “Beta” in the web interface. And as @buzz stated, you can later upgrade to the RC and once it is officially releasd to the stable version. Make sure you switch the “Release Channel” back to “Stable” once the stable Version of NC 21 is released and installed.