Not sure of the answer to yor questions, but here's an option you might want to consider. ( many of us do this ) We render are images while were at work or sleeping or not using the computer. There's an option that lets you save a partially rendered scene so you can open it and resume the render right where it left off. Go to Edit/Preferences and enable the Image with Scene Open/Save option. Then when you save the file a bmp is saved with it. When you open the scene back up, simple hit the resume render button ( little button just right of the render button ) Hope this helps.

Stephen Ray