One remark from my side:
1) I have managed to make it work. I had to download all multiverse components directly from Bukkit site (NO SPIGOT!) after a long research it turned out that all multiverse information on Spigot site is incomplete and outdated. I had to manually upload multiverse-core and multiverse-portals found on bukkit via file manager. They are not for Spigot but they work ok with Spigot.
2) Main reason for me to buy a licence of AMP was to have a kind of curated, tested environment to effortlessly install 2 minecraft servers. My hope was, that I will be able to more or less plug-and-play (at least fully from GUI) install a simple multiverse server, add basic plugins and import worlds. My imagination was, that AMP is a kind of environment where you can configure everything from GUI, including the plugins which are tested to work with the server.
What i found is that even on a builtin AMP list of plugins there is a huge mess. There is a bunch of incompatible with each other options of Multiverse, with no clear indication which should be installed for each version of Minecraft server etc. This did not help me at all.
To make things worse: I could NOT ununstall from GUI the plugins I did install from GUI. The uninstallation did not work at all. Maybe because I did install 2 different multiverse versions but I am not sure this is the case. I did ssh to the server and manually "rm *" the plugins.
So my overall feeling is that the purchase did not achieve the main goal. Treat it as a user feedback to take into account when planning a roadmap for next moves.