You can use the References pane to associate any document with any other document.
True, and there are no plans to add them either at the moment. Scrivener is dedicated to writing and isn’t intended as a general information manager or database program. The fields available are closely integrated with the inspector.
Hopefully one of the MMD users can help you here as I haven’t done this myself, but basically you need to use the MultiMarkdown plugin for OO to export from there as an MMD file and then use Scrivener’s File > Import > MultiMarkdown File.
The general idea is that you add your own synopsis that is more meaningful than anything that could be automatically generated, but you can use the auto-generate synopsis button in the top right of the index card in the inspector. This fills the index card with the selected text of the main document or with the first few sentences if nothing is selected. (The first few sentences get put in as the placeholder when importing, too.)
Keith has described the procedure you would use to export from OO and import into Scrivener. As far as I know, the MMD plug-in for OO is export only. The designer built it to work as a front-end for creating complex MMD documents, but has since switched to using Scrivener and TextMate for all of that.
Everything you need to know about the plug-in can be found in this document.