> QUESTION: Does resuming work with instances?

Resume works with instancing, you only need to reload MXS and select resume before you hit [Render] again.

So,  avoid stopping and resuming in the same session to stop instances disappearing gradually.

> By enabling Motion Blur I don't get instances but copies.

Motion Blur and instances cannot currently run together in Maxwell. So if Motion Blur is enabled in Maxwell Render Options, the plugin will automatically treat instances/copies as standard geometry, allowing the render to show the desired motion blur (but not having the memory benefit from instances).