I know of Sekiro and Khazan so far - Sekiro via its reflections and Inner variants, and Khazan via just repeating the boss fight where you fought them initially. I quite like both and think it’s cool to re-experience the fights if you feel like it. Which other soulslikes do that?
You can. Requires an item and puts that specific area on the next ng cycle.
Item is relatively rare.
Except for the part where you can farm them infinitely in the giant memories.