it was never stated that curses make the user insane, that only happens for a few, the reason Rupin went insane was because of his past combined with the Fire Curse having a tendency to make its user more aggressive and impatient, since the Curse alters its holder’s mind to take on the properties/emotions of the Curse
devil fruits if they altered your mind, made you immortal, could even control you, take part of your soul, and be passed on through generations… so no, not really. The original idea of them was based on devil fruits, yea, but they are a thing of their own now. Just calling them devil fruits really dumbs it down and causes confusion
as for this thread, nah the final boss of the bronze sea won’t use a curse directly