The envelope/document model can only be used with an Xml message. You would still have to convert your flat file into Xml using the Flat file disassembler, so any error in the flat file would result in the whole message being discarded.
If you receive Xml to start with, then the Xml Disassembler only requires that the Xml be well formed. If it is not then the whole message is discarded
Otherwise it will break down the message into individual documents. The Xml Disassembler does not validate a document against it schema. It just uses the namespace and root node to identify the schema. If you wish to validate against the schema you need to use the Xml Validator pipeline component in the next stage of the pipeline. WIth Recoverable Interchange processing neither error (cannot find schema or fails schema validation) will prevent subsequent valid documents from processing
Things are much easier using Xml – probably one of the reasons for it popularity