Kinda had mixed feelings on that at first, but considering the potential that this has, this might be interesting. As for the reason, I think it's not that weird, as she fell into the water and was literally drowning because of her Devil Fruit. Pretty believable that the brain gets "damaged" in the process.
I think this situation might lead do another step or potentially even closure to Big Mom's backstory? Depending on how Oda handles this, this could be pretty interesting.
At first, I was worried about Sanji's situation. Him using the suit kinda goes against what his arc was about, becoming strong on his own and not be like his family. But I think, he already proved to his family, that he managed to succeed on his own. Especially considering that Sanji is shown to have an inner conflict while using the raid costume, makes it acceptable to me. Him saying that he will throw his pride away in order to protect his friends, is very in-character for Sanji and makes the use of the costume believable. It's just a shame that Sanji never really got to shine since the time skip and gets a power boosts through science. Of course he got a huge arc dedicated to him, but fighting wise he always drew the short straw. Could have been handled better.