Hi Bill, here's some answers for you.
1 - if your array has internet access yet you cannot see the new 2.1.x firmware then there's a good chance that your system has been blacklisted due to a known bug or problem that you will encounter during the upgrade. (You can also verify this by heading to the assets tab in Infosight and clicking the OS version your system is running). It's worthwhile giving support a call to find out what the problem is and how to fix before upgrading - please do not manually upload the firmware to your array as this could have severe consequences.
1a - yes, the single firmware image is compatible with all Nimble models.
2 - it's advisable to unregister and re-register the plugin if possible before & after the 1.4.x to 2.1.x upgrade.
3 - the array will upgrade the system to ensure no downtime is noticed by your applications - so once the standby controller has taken the update & verified all is good the controllers will perform a handover (typically 3-4 packets dropped in this process), and will then proceed to upgrade the second controller. The controllers will not fail back as there's no need to do so.
It's worth taking some time to install Nimble Connection Manager for Windows & VMware in your hosts before upgrading to 2.x also. This will require a host reboot (Windows) or Maintenance Mode (VMware).
Hope this helps, I would definitely reach out to support to get them to look at the blacklist before taking any further steps.
Also - if you don't see the code you want available in the downloadable code screen you can call support or you can login to infosight. In infosight you can choose the "dashboard" tab. Then at the bottom of that tab is the "Upgrade Needs" section. If you highlight the click on the "Current Version" of the OS on your array - it will show you if your on a black list, and what steps steps would be needed to get the latest release of code.