Morgan newspaper=/=how WG decided Luffy's bounty. Morgan called Luffy 5th emperor. Gorosei never referred Luffy as 5th emperor.
Destroying BM castle did not contribute to Luffy 1.5 b berries. Franky destroyed SAD he still got 50 m berries increase after Dressrosa.
Morgans got his scoop about Luffy having a grand fleet from Stussy. Stussy is from WG so WG already knew Luffy grand fleet yet they did not boost Luffy's bounty after Dressrosa.
Shueisha confirmed Luffy got 1.5 b berries for solely beating Katakuri