Everyone is pretty responsive on the mailing list and you can also feel free to hop into #mongomapper on irc.freenode.net.
Yes. As of 0.9, Active Model is used.
A Document is a top level entity that has its own collection. An EmbeddedDocument is embedded inside of a Document. For example, in the document below, Template would be a MongoMapper::Document and would have a filename key. Field would be a MongoMapper::EmbeddedDocument and would be stored inside of the template document as an array of fields.
The general rule of thumb is to embed when the document will only be shown in the context of the parent document. If you need access to the document outside of the parent, don’t embed. Instead de-normalize and store the document in two places.
Different strokes for different folks. Read up on all of them and give them a try.