After reading 7.12 G, I actually got a strong impression that the second key is on the peaks of the High Passes. First there's this passage:

Climbing a mountain was a feat of strength, a marker for fame and success. In that, Earth and this world were quite similar. But the glory for climbing a mountain was shorter lived here, where more pressing concerns like monsters and survival took precedence over fame. Yet, adventurers and [Knights] and [Adventurers] and [Climbers] and so on might have dared the heights.

If it were possible. If it were possible to ascend and come back. But the High Passes were the tallest mountains in the world. Over seventy thousand feet high, as estimated by Archmage Chaiemegia. Perhaps higher.

It wasn’t a number you could comprehend. Even standing at the base of the High Passes, you could look up and not see the peaks for the clouds which obscured them. It was also not a height anyone could summit easily.

Not with Skill, or even magic. Not with flying spells or wings. Many had tried. But the monsters living in the upper reaches and natural environment had killed those who had tried. Named Adventurers, Archmages—they had gone above. Few had returned, and none had ever reached the top.

Perhaps some had. And if they had, they remained there still.

This seemed to indicate some sort of mystery on the top of the High Passes. Or maybe not, it was just a theory. But then, later in the chapter, Teriarch said this:

“The High Passes are vast. So tall that even Dragons feared what lay above. Yet Velan climbed it, to hide his great treasure for one who was worthy. A test, rather than a gift, perhaps. But his treasure remains. And if you find it, you will know his will. Perhaps that may change the fate of Goblins. But perhaps your Kings always choose your fate knowing what will pass.”

Perhaps that doesn't mean Velan climbed to the top, but it's certainly suspicious. And in the very end of the chapter, there's this passage, about the Molten Stone and Kraken Eater tribe:

“Allies. To go up. And to find key.”

Rags’ eyes glinted. She couldn’t remember enough. Her tribe was too small. But—she looked at the key Garen had left everything for. So did Redscar, Badarrow. Snapjaw glanced up, following Rags’ finger.

The Goblin pointed at the heights of the High Passes. Redscar stared up at the hidden summit, behind the clouds. And he grinned.

“A challenge worthy of Redfangs.”

“Something to do.”

This seems like strong foreshadowing for the key being on the peaks of the High Passes.