I don't think there's any way that King Grayskull and He-Ro can possibly exist in the same canon, which for me, is ok. I gave up on trying to merge the canons in my head long ago. Even in the 80s, there were multiple canons for MOTU, and that was ok.

Everyone chose what they wanted it to be.

The reason I don't think that KG & H-R work together is within the Elders themselves. In MYP continuity, the Elders come into play after King Grayskull's death, and Castle Grayskull predates them.

However, in classic continuity, the Elders are the ones with the power, and the entire Hall of Wisdom is what becomes Castle Grayskull.

MYP had the Hall of Wisdom and Castle Grayskull as seperate entities.

You could somehow put King Grayskull in there somewhere after the original Hall of Wisdom becomes Castle Grayskull, but it seemed in the MYP cartoon that before King Grayskull, Castle Grayskull was just an empty castle with nothing magical about it.

It makes my brain twist in knots just trying to think about it. I'm much happier having them exist in an Elseworlds style. Two different genres.