Hi,
For your first request. I can recommend you to use "Region select" in VFB only to see updates in a particular region when you are making some changes to the material of a particular asset. You can also use the "Render selected" option present in the Scene tab of the Corona render setup to only update according to the preset you select from the "Render selected" dropdown.
For your second request, I am not sure what exactly you meant because I am unable to reproduce a change in the VFB resolution depending on if the second monitor has a different resolution. Here is a recording :
https://bit.ly/3cfBYjRYou can also use the resume last render option present in the scene tab instead starting all over again. (Depending on your need)