See, I was a kid when I saw it, and none of the things you cite as being "too much for a kid" line up with my personal experience with the story. I was perfectly able to grasp the idea of "The Nothing" as well as ANY human being with a limited capacity to perceive infinitely scaled concepts like that. I was even able to grasp the idea that it was a representation of his memory of his mother, and his childhood/innocence, which he was losing as his mother's memories faded for him. And that him imbuing the princess with her name, was his way of coping with her loss, and a way of immortalizing her in his mind, by tying his memories of her, to the realm of TNES, as an emotional, and mnemonic anchor.
I dunno, I mean none of the films I cited for being intense, were "too deep/much" for me seeing them, as the little 8-10ish year old I was. I got them just fine. I wasn't confused by The Nothing, but I was scared of it in the framework of the film. I wasn't confused by the rock giant crying over his lost friends, or the horse, I was sad because the film WANTED me to be sad at their losses. Being sad isn't alien to children. Sadly, many of them are all too familiar with the various things being hinted at in these films, due to IRL. Like I didn't want to know what it was like to have my older brother choke me out and threaten to kill me when I was 12ish years old, but I do know what that's like. I wish I didn't know what it was like at that age, to have parents who constantly yelled at each other every night, to the point I had to blare loud rock music in my headphones at night, just to drown them out, but I do. To see those things in a film, it doesn't shake my foundation, it's just a film.
I just think that those elements are not as foreign and strange for children to understand, depending on how they are framed. But again, that's the whole point of the G in PG. It's GUIDANCE. It's flat out stating that this film, will touch on topics that may, or may not, be confusing for your child, depending on their specific age and comprehension of various aspects of life. So be sure to watch it with them, and be ready to discuss the issues if they come up. PG doesn't mean "This won't challenge your child in any way at all." And given how long PG films have had "intense" subjects, I'd wager it never has meant that. Which again, is why they have the separate category of G. To specifically say "This isn't going to make your kid cry. NOTHING in it is at all scary or challenging for them mentally, emotionally. It's ALL puppies and rainbows."