Thank you for clarification and sending your simulation file, I have a better understanding of what you are trying to simulate.
As you said, if you are planning to look at farfield profile, adding any other material to your simulation will affect your results. Here is my response for your questions:
No, these two sources will behave independently. You can take a look at this KB example to learn more.
In Lumerical we can export a custom beam profile as explained here. However, the approach for creating a Gaussian source with oval shape will be challenging as you need to set it properly so that it is a focused beam. If this is what you are planning to do, we can work on it together.
I don’t think we can add a material that completely blocks the field and gives no absorption. Every material will give a reflection (unless it is dark matter!), and unfortunately we don’t have artificial material to use in our simulations to do this task.
These are different ways of setting your Gaussian source. For example, if you know the beam waist and offeset you can use scalar approximation, and if you know your lens settings in an experiment, you can use thin lens which you will need your lens NA and distance from focus. Please refer to this page for more information.
If you are only interested in reflection from your gold geometry and a Gaussian distribution of field on gold is not important, I will recommend using TFSF source. We can set it in a way that it covers only the metal section and capture only the scattering light from metal section.
Please let me know what you think and I am glad to discuss it to come up with the best possible solution for your problem.