That was a bunch of good catches and suggestions! I fixed the issue with the crown - now you have to defeat the enemy before you can grab it. I added a new state to the Journal once the player has seen the Queen's Harp among the merfolks' wares. The link for burning slimed goblins now disappears after you've burned the last of them. I fixed the wording with "one goblin left" too. I didn't change the link text to "burn the last goblin", though - I agree it would sound better, but the link text is pulled straight from the enemy object, so changing this would require a lot of fiddling.
While fixing the Devourer stuff, I also realised another issue - if you escaped from that fight after burning some goblins, then re-entered combat, the number of goblins would be reset back to 5. This is now fixed - once you've burned a goblin, it's gone for good.