Can I safely load previous save files?
You are allowed one manual save and five auto-saves. The auto-saves happen rather frequently, by time interval, progression, and travel.
If you still have a save before Eventide, it would have to be the manual. If you loaded that the auto-saves would likely wipe out your current progress before you'd complete the island.
Save files on the Switch are tied to each console and cannot be transferred, however it might be possible to back up and restore saves on the Micro SD card by using a computer. I wouldn't want to be the one to test this theory out though.