The current Jira will be used (with new sub-tasks if needed) when we'll start working on the MP forms or when we'll have at least wrote the specifications of those enhancements and planned to work on them.
I have created a Jira about updating the MP documentation which is really out-of-date: NXDOC-122. For now, the best documentation is to take example on the nuxeo-marketplace-sample project.
About the MP forms, they are not working when their container package is installed in offline mode (Nuxeo server shut down). If you write a MP which declares itself as hot-reload-able and install it from the Admin Center (Nuxeo server up), then the forms must still work (else it's an issue we'll quickly fix). Even if the package is not "truly" hot-reload-able (some bundles or services not available until the server has restarted), a server restart after a hot-reload install of the MP will work, including the forms parameters being properly saved.