how to downgrade an unknown (2005) interface name from SSIS 12 to 05
In SQL Server 2005, there is an IDTSConnectionManager90.  It comes with the note "this interface supports the SQL Server 2005 infrastructure and is not intended to be used directly from your code."  A very similar note is provided for IDTSConnectionManager100.

These two are not 100 percent identical, of course.

However, these are SQL Server version specific components and are intended to be used through the ConnectionManager class.  For 2005 see:

http://technet.microsoft.com/en-us/library/microsoft.sqlserver.dts.runtime.connectionmanager(v=sql.90).aspx

http://technet.microsoft.com/en-us/library/microsoft.sqlserver.dts.runtime.connectionmanager_members(v=sql.90).aspx