hey,
Here are the answers per me:
1. While configuring the dynamic media(DM) in the cloud configuration, you need to mention the delivery time to DM(upload or activation), In your case activation make sense as no need to do re-upload or run DAM update workflow where scene7 uploading happens provided you selected upload option while configuring.
2. DM stores only a single master file PTIFF which generates the dynamic rendition based on image presets. No static rendition will go to DM.
3. Do not delete static rendition as these are used in the authoring interface depending on the location(tile size etc). Let static rendition be there unless there is a very compelling reason to get rid of them.
Hope it gives you some pointers.
Thanks!