It's not a factual error but who on earth would be stupid enough to go hiking in a forest with no map, no flash light, no hiking shoes or protecting gear?
Japanese forests = mosquitos, leeches and mukade (centipede)
and on a mountain, it's cold and damp, and she wears a hoodie? When the guide said they'd come back tomorrow, she should have gone. she's an idiot. No proper clothing, blankets, food, water, medical supplies. Even if it's just for a day, you should be prepared. Anything can happen.
It's not the size of the dog in the fight, it's the size of the fight in the dog

I knew it was going to be crap when they said they put their grandmother in a nursing home to start a new life in Japan. Asian people would look at you like you killed a kid if you told them your grandparents were in a nursing home. In China they go as far as public shaming people who are not good to their family elders.

You know you're talking about a cookie cutter Hollywood horror film, right? It's a movie, it's not real life, so all those things you mentioned don't really matter. It's not supposed to portray Japanese culture or something.

Good luck with that. My gps wouldn't work for crap in much of Japan

Also, GPS on a phone works adequately well even if there is no cell tower coverage, and even in a forest as long as there are occasional openings. I have a few apps on my phone which would make it a doddle to navigate around that forest. Unless the evil spirits can block GPS signals of course

Would a modern society allow a "suicide forest" to even exist?

1. Japan is not a "modern" society, as evidenced by the superstitions, etc.
2. Forests just are, no one allows them, people just go in there to kill themselves.
   It's not the size of the dog in the fight, it's the size of the fight in the dog

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You do know the suicide forest is a real thing though right? Or maybe you're just trolling.

Would a modern society allow a "suicide forest" to even exist?

Once they were at 'The Forest', they were filming in Serbia. Which is probably why they filmed elsewhere than Japan (from what you say, it makes sense).
While at 'The Forest' scenes, all cars are left drive; if they really were in Japan they would be right drive. They didn't even TRY to cover up that obvious blunder.
They were going in for the whole day and they hardly took any FOOD/Water along with them.
Whenever I've gone out for a day hike (western USA), I take food (& water if I know I can't get any) for 2 or 3 days. That guide Michi should've known that much - and left them some extra.
But if it's all an illusion, it doesn't matter.