Hi Dorvan,
Plugin manager
Plugin Manager is not part of Roundcube core. It is a management tool for roundcube plugins. The goal is to help Roundcube admins maintain their plugin set, ensuring the plugins are compatible with the version of Roundcube installed plus several additional functions.
(that seems to continuously have a lot of problems)
Can you mention one? Have you inquired our team regarding any issues with plugin_manager? We have stats of several thousand servers running plugin_manager on daily basis issues free. We would be certainly interested in hearing from you some of the problems it has. It will certainly help our team as well as many others to iron out issues anyone may find.
ask for a credit (why 
) to update it or download something.
MyRoundcube offers free and non-free downloads. Downloads are provided through a credit system. From more than 40 plugins currently available in the mirror servers, many of them are provided as free download. Plugin Manager helps you download the correct version automatically. It also checks for plugins updates in our mirror servers against your current installed plugins. Localization updates are also made available through it.
how it's structured the project ?
We are not part of the core Roundcube Project as Skaero mentioned.
