1, The schema is a message specification. It specifies the message type (targetnamespace#root node). It can be used to validate messages in the pipeline using the XmlDisassembler or XmlValidator pipeline components. It contains the the specifications for promoted properties and distinguished fields. It contains the envelope property thst controls pipeline debatching. A flat file schema contains instructions for the Flat File Assembler and Disassembler to parse or serialize a flat file to/from Xml. An EDI schema does the same for Edifact, X12 or EANCOM messages….
2. There are many examples of complex schemas . e.g UBL http://docs.oasis-open.org/ubl/os-UBL-2.0/UBL-2.0.html and http://docs.oasis-open.org/ubl/os-UBL-2.0/xsd/
3. The message context contains promoted properties and distinguished fields. The promoted properties can be used for message routing where as the distinguished fields are only available inside an orchestration ( or custom pipeline code). Each promoted property is limited to 256 characters while a distinguished field has no limit. The MessageBox pub/sub mechanism relies on promoted properties without them it would not work.
4. I am not sure promoted properties do consume more memory than distinguished fields e.g the BTS.MessageType promoted property value is typically less than 100 characters, whereas a distinguished field could contain megabytes of data.
5 . A property schema is where you define your promoted properties. The quick promotion button is just a short cut to creating the property schema. You can define your on promoted properties that can be used for content-based routing and controlling the pub/sub mechanism central to BizTalk.