First, you need to understand that they are not here to settle on something fair, they are here to get all they can. Let me try to explain why you are asking for what you asked for:
1) The image is properly registered. - This determines how much they might collect in court. Maximum damages are different for a properly registered image than a non registered image.
2) The have the exclusive rights to the image. - This determines if they have the right to ask for anything. Only one entity may hold the right to pursue copyright infringement. This is usually where the trolls slip up. That's why they usually avoid answering this question.
3) The provide a signed agreement showing the owner of the work has given Getty the right to collect monies on behalf of the artist. - This agreement will tell what kind of relationship Getty has with the artists. If the artist did not transfer full rights to the image to Getty, Getty cannot pursue copyright infringement and win.
4) Show sales for the last 24 months on the image justifying the amount they are requesting - Generally a court will ask this to keep their demands in line. They should be asking for something reasonable.
I am interested in this part of their response
A rights holders form has been attached for your reference."
What is this rights holders form and what does it say? Is it signed and dated by both Getty and the photographer? If not, it may not hold up in court.
You can play the letter game with them, if you like. It may ease your conscience. Generally, it results in them pursuing you harder because it lets them know you have a conscience. If you choose to play the letter game, be careful that you do not reveal anything that may be detrimental to your cause should this thing escalate. It likely won't.