1. The entire length of your new reservation needs to be available because Disney changed their methodology and it is no longer easy to just add/drop nights. If the entire length is not available you might have to consider making two separate reservations to cover all the nights.
2. Depends on if you booked with a special offer. If it is a discounted rate all the nights have to be included, there needs to be availability and the special hasn't expired.
3. You will have to contact the agent because they own your reservation and Disney will only talk to the agent.