How to pass the node id from a View Block to an associated Views Attachment for use in filters so that both operate on same content node id?
Since the attachment is linked to a specific view block, it should gain access to the information in the block.  You can inform the block of the node ID by using contextual filters.
1. In the block  add a contextual filter of the type content:nid.
2. Since blocks do not have paths you must explicitly tell the block to find the display node, so under form heading of "When the filter value is not available", choose "Provide default value" and select "Content ID from the URL".  This should provide the Node ID to the view block.