In fact you dont need to beat every boss to access Inkwell hell. You need to beat all to access King Dice's fight, but to get 12th coin you only need the Phantom Express and bosses that opej up path to him beaten
If you found all the secret coins and talk to the coin dude from inkwell isle one he will actually have another dialogue and will say something like "so you gathered all my coins eh? Well finders keepers that my motto. Now run along little mugs before I change my mind!"
I have a question how come I can't buy every item I have gotten every coin beside inkwell hell but I have collected 39 coins but still have three items I have not bought yet