Installing UniFi Controller on a FreeBSD jail is pretty simple, even if there’s no plugin for it. All you have to do is create a bare jail, access it either through its console or ssh, and install the net-mgmt/unify5 package. You might have to look into enabling the default FreeBSD package repository for that, I don’t recall off the top of my head, but other than that you’re literally just a single ‘pkg install’ command away. I created my UniFi Controller jail like some good months ago and it’s been running like a breeze ever since.
On the other hand, you might get an outdated unify5 package, for which the only solution I found was to create my own poudriere builder in a Bhyve VM, update the port there and build it, and have the jail fetch the resulting packages from it. I’ve sent my updates to the unify5 port to its maintainer to have my changes trickle into the default FreeBSD pkg repo, but unfortunately he hasn’t responded.

Hmm, it should be grabbing the 11.2 version of the plugin. If the issue persists after updating the OS to 11.2-U6, create a report at bugs.ixsystems.com.

Yes, Collections is a new feature coming in 11.3:
Click Browse a Collection to toggle the plugins list between iXsystems plugins, which receive updates every few weeks, and Community plugins.

Instructions are in the New Plugins for Testing section of https://www.ixsystems.com/blog/february-plugin-updates-and-new-plugins-for-testing/.