You are confusing “pamac/add or remove software” with “nvm”. NVM (Node Version Manager) handles different versions of node. You cannot use pamac/pacman to do that. Arch only supports the latest version of node available and so does Manjaro. So changing pacman.conf
doesn’t matter.
Versions installed with nvm cannot be updated with pamac. See stackoverflow on how to properly update those.
Tl;dr - What you are trying to accomplish is not possible with pamac and there is a reason why tools like nvm exist. Install a node version with nvm and make use of .nvmrc. Also see Node.js - ArchWiki.