I redownloaded the debug build and have been playing around with it again during the new lockdown, now knowing I could actually drive around in Niflheim, and managed to do it.
Later I explored other options, tried with "Reveal all map icon", and found some dummied NPCs/Warp Points at Hammerhead, one makes sense as internal logic of the game, the others not so much, unless they were simply there for testing stuff. These icons only appear on regular version of Lucis, they don't appear on the World of Ruin version.
These are: 1.- Insomnia | 2.- Lunafreya | 3.- David (The hunter npc) | 4.- Magna Fortia/Succarpe | 5.- Shiva | 6.- Ardyn | 7.- Iris
I marked them on the map directly on their icon, to check exactly where they were supposed to be.
1.- Insomnia | (in place of) Caravan: Makes sense I guess?, is the last camping site you have access before going to Insomnia near the end of the game.
2.- Lunafreya | (in place of) Door: yeah, she's a door.
3.- David/Hunter Npc | (in place of) Shop owner: don't remember seeing him there at all during the campaign.
4.- Magna Fortia/Succarpe | (in place of) Cindy's truck: now, this is interesting, because Cindy's truck is actually on the Succarpe region of Niflheim just hanging out there. Check next set of images
5.- Shiva | near the garage
6.- Ardyn | near the garage
7.- Iris | (in place of) Weapons vendor: again, interesting, I didn't play much of the multiplayer but I do remember Iris becoming a vendor there, repurposed idea maybe.
Here is Cindy's truck at Succarpe (this isn't news, it was known for years her truck was there for some reason, some people with normal retail version of the game also had the icon appearing at Hammerhead, not sure what would be the trigger, I never saw it playing normally), you may not remember what exactly "Magna Fortia" is though, well, that's the name of the train that brings you to this region. During the day there are no signs of a train here, but at night some train lights appear.
Succarpe (written as Sccarpe) is the only warp point from Niflheim available on the debug menu, everything else is for Lucis. So if this area was supposed to be playable, it was cut fairly late in development, probably their last hope of having some sort of mini open world section in Niflheim to more seamlessly bridge the gap into the linear sections.