For delrin settings looking here would be a good place to start
https://community.glowforge.com/search?q=Delrin%20settings
For the second question you machine cannot engrave metals, you can only mark onto them with products like Carmark or coat them and remove the coating with the laser. There are some instances where you can mark directly onto stainless steel but I have not done this myself and it does not take any material away.
For the leather you will need to run a material test of some sort to find the most ideal settings for you.
Here is one that was posted to the forum.
Do you have a photo of the kind of burning you are trying to avoid?