Scheele was not a British Chemist but from Swedish Pomerania like Blucher.
Scheele Green is a mixture of
This not in itself toxic as it is not bioavailable. The production of Methyl Arsine produced in a damp environment would make the Arsenic available in the vapour form and hence cause poisoning.
Green was Napoleon's favorite colour so he chose the wallpaper. He must have used the colour in his other palaces. Napoleon was already ill before going to St Helena.