Replicating data between SQL Server 2014 to SQL Server 2005 - temporary solution
Reading your question and thinking a bit more about a possible solution leads me to recommend transactional replication.

Make sure you do implement a change freeze to your schema. 

From BOL :


  For transactional replication, a Subscriber to a transactional publication can be any version within two versions of the Publisher version. For example, a SQL Server 2000 Publisher can have SQL Server 2008 Subscribers, and a SQL Server 2008 Publisher can have SQL Server 2000 Subscribers.


Based on above truth, 


Take backup of your current sql server 2005 database and make sure you have instant file initialization enabled on all servers.
Freeze all the data and schema changes &lt;-- This is your downtime.
Migrate your database from sql server 2005 to 2014 and do all post restore steps.
Repoint your application to the new server instance.
Set up T-Rep from sql server 2014 to sql server 2008R2 (this will be an intermediate server or instance and can be on the same new server if you want).You care about this step since, you want to be on supported version. Make sure you initialize replication manually using tsql.
Now have another layer of replication from sql server 2008 R2 (intermediate) to your sql server 2005 server. Make sure you initialize replication manually using tsql.


Note: If the servers are in same datacenter, then you can even do a snapshot which will be very fast.

Above exercise will be worth, since you want a supported rollback plan. Since your business wants to keep things around for 2 weeks, you should be good once you test above.

Note: I have not tried (as I have not been in a situation wherein I have to replicate data from sql server 2014 to 2005), but it should work (though not supported) theoretically. Give it a try for small database and you will easily be able to gauge the effort required.