So, what is the pont of the battle with Calder?

[Metastachydium]

He will reveal some crucial backstory on what really happened between Soon and his team.

I'd phrase it more like "between the Scribblers", honestly, but yeah.

I think it's to convey the severity of the power gap. Calder is probably going to be quite a challenge for the group, which means they're in turn not necessarily ready for Team Evil. If they can't readily crush an old red dragon with lots of power and domination magic at his disposal, how are they going to take on an Epic sorcerer, a cleric with 9th tier spells

It's also a drain on party resources.

I'd argue that goes both ways. A crucial thing that bears mentioning is that, at the end of the day, Calder is really just a line of defense in that dungeon who happens to be hostile towards the Good Guys… In a dungeon with many lines of defense equally hostile towards the Bad Guys.

In that way, it might also help demonstrate how tough this place is while leaving the camera focused on the heroes doing the cool thing that is soldiering through it anyhow.

And lastly: the story is built on a Dungeons and Dragons shell.
Fighting a dragon is a thing. It's a good thing in and of itself.

And fighting a dragon in a dungeon… Yeah. Hells, yeah, in fact.

I am still puzzling out what Eugene's return (convo with Roy) folds into at this stage of the game.

All the Dwarves in the world will suddenly appear and throw themselves at Clader dying in droves, probably?

an unknown creature with enough raw power to make earthquakes at will?

How do you know it's at-will?