[@Bishop]
Originally Wisps were going to be more spirit like, but then I figured "No they still need to be at least partially physical cause the others need to be able to hurt and kill them."

As of now, a Wisp DOES have a physical body. It is essentially a Koosh Ball (look it up if you don't know it but I bet you do and don't realize it) made out of very squishy flesh and soft hair instead of rubber. The hairs have a quality somewhat like the fibers or filaments used in electronics, and the magical energy that suffuses such a small body flows through them to create the differently colored auras that make them look like floating magical lights.

If they use up their MP, they have a much harder time Levitating, just like you would have a hard time holding up your entire body on a pull up bar when all your muscles are exhausted. They won't generally run out of MP just FROM Levitation, though. If they can't hold themselves up and they fall, or if they are knocked to the ground, they DO take fall damage.

They do need to eat, sleep, poop, etc. It is assumed the less pleasant biological functions will usually take place off screen. So long as they are near some source of food and water they don't have to get very specific with that either. Pixies and Wisps can, to an extent, subsist off of Mana Crystals and ambient mana, but that's mostly like baby food or something and won't keep them going indefinitely once they mature enough or start using up more energy to hunt and fight.

Wisps are NOT anywhere near equal to human intelligence under normal circumstances. I'm not sure what else you're asking when it comes to the Earthlings' minds overwriting those of the monsters they have become. Even if there are "stats" mentioned, this isn't a setting where one can be smarter than someone else just because they have a higher Intelligence or Wisdom stat. Learning is based from experiences and study just like in the real world. As former humans who have already lived and been educated in a modern day environment (except a few characters) the players are far and away smarter than most entities of this world in regards to certain things.

Write the first few days however you'd like, but in the first week there aren't any random encounters because the characters are basically infants during that time. If they didn't spend their time hiding, avoiding conflicts, keeping to themselves and only getting what they needed to survive, etc they'd already be dead. If you want to fight certain creatures just look specifically for those creatures in the cave and you'll be more likely to have a random encounter involving them.