How do you explain that?

I explain it by pointing out that you're completely wrong, and that panel doesn't make Law look anywhere close to Trebol's height. Even accounting for the fact that Law is at an angle to our perspective, he looks barely more than half Trebol's height.

I don't want to make statements as if they were fact when all we're doing here is speculating.

And the problem is that you're expressing confidence in unfounded speculation. Speculation which is, in some cases, contrary to the established principles by which things in One Piece operate.

Marco is the only one of Whitebeard's men specifically mentioned by the Gorosei within the manga to be a real threat to Blackbeard

They mentioned him as the leader of the remnant of the Whitebeard pirates. And it doesn't take the strongest haki on the planet to hit Blackbeard; it doesn't take haki to hit him, in fact.

What stops them from redistributing their core logia body mass, for example making so that their chest consists only of just magma, not their bodily essence anymore, while pushing their actual physical logia body into the rest of their body and making it denser?

The fact that we were told by Oda in an SBS that when a logia user creates their element, whatever they create is part of their body - that they can't simply create a projection of their element. And the conversation between Vergo and Smoker during their fight about what happens when a logia fights someone with superior haki. And the cases where it looks like a logia created an element which got hit using haki without getting injured are all ambiguous... which leads back to relying on what the author has specifically told us.