Good chapter.
I find the money strange. Here is chest full of ancient coins, many of which have to be in better condition than any found in museums. Wouldn't most of them be priceless? And yet they seem to be so little in Diagon Alley. Did the Goblins cheat Harry?
And the White Elephant. It was not unique to Thailand but was common to all of Southeast Asia. Your presentation left out that it was (still is in some places) considered sacred. Its presence indicated peace and prosperity for the kingdom and that the king was ruling with justice (a bit ironic there if a malicious gift). So while its expense with no benefit (you didn't explain that as a sacred animal it couldn't be used as a beast of burden) could be a curse, it would primarily be considered a blessing -- a mixed blessing indeed, but not merely an attempt by the king to ruin someone. And that's how it is used today -- after all Aerospatiale never intended to lose money on the Concorde.
Was it right to have the old man cheat Harry (and Pim) by taking twice what "the White Elephant" (perhaps that should be the huts name) is worth?
Why does Dumbledore meet with Fishman the last day of August, the day before students return, but then Harry has two more days left before going to Hogwarts?
I had the impression the fishman died when his "home" collapsed on him! Glad to see he lived.