On your first question you said the agreement was created for wrong customer. Since you already baselined the funding, I assume you have that customer linked to the project.
You can go the Agreement form and enter a negative funding line allocated to the same project. The total allocated amount will be zero. The agreement is now not allocated and free to be allocated to another project. If it is a wrong agreement you may update the agreement amount to zero. Now create a new agreement for the right customer.
The open issue is with the project. You have a wrong customer associated with that project. However, system will not allow you to remove the customer - project record. Solution depends on the way the project was setup. Did you enable customer at top task? if so, end date the top task and create an alternative top task with the right customer.
If the project has been set with a single customer you better close the project and start a new one.
However, if you need an alternative solution, I can suggest a custom method. Go to Help > ..> Examine and change the existing customer of the project to zero contribution.
Now you will be able to add the right customer to the project and have 100% contribution on it.