Those aren't SATA 2 speeds, so definitely bottlenecked.In the manual, the block diagram does say SATA 2.
Cloning the UEFI GPT based drive to the MBR based drive expecting AMI isn't going to work. You'll need UEFI for both to just use one or the other if you want, and it'll require your bootloader USB at all times.The SATA SSD will be kept as MBR so that I can still boot in the event something happens to the NVME drive or thumb drive with the bootloader on it.
No it'll create a backup image of your boot drive to the SATA SSD. If something did happen to the NVMe drive you'd need to boot into a Macrium Reflect USB drive and tell it to restore the image. What you're wanting is cloning which won't be possible with how you want to implement the setup, or basically a shadow copy if you will as redundancy.So using Macrium Reflect I can tell it to just backup the Windows partition of the NVME drive to the Windows partition of the SATA drive and still be able to boot from the SATA drive if needed?