I'm pretty sure Saharah and Gulliver are the same. I've seen them available quite late.
The only NPCs that I know of that have nighttime limits are Daisy Mae (6am I think-noon) and pre-Able Sisters Mabel (not sure when she starts, but she packs up at 10pm, same as Nook's Cranny).
I don't know when the earliest you can encounter Wisp is, but I know I've seen him before 10pm. I think I might've even seen him as early as sometime after 8pm.