Nope, that won't work. Compatibility mode has nothing to do with the database version.

You cannot restore back to SQL 2005 from 2008. Your only option is to script all of the schema's from the 2008 databases into SQL 2005 equivalents and then import the data using something like SSIS.

You could reduce the amount of leg work by using a 3rd party tool to compare and sync schema and data between databases. There are several vendors that produce software of this type including Idera and Red-Gate.