I wonder if knowing the enemy were Fishmen played any part in Bellemere's decision? Even if Nami and Nojiko could have hidden on the island itself for a while, sooner or later they'd have to leave by boat, and then they'd be more visible - and to such an enemy, especially vulnerable.

eating a poisonous mushroom when knowing that it is poisonous just because your pupil found it for you

Well, he was also dying in any case - he would only have had a few days left to live even if there had been no musroom soup and no Wapol plot.