cake with banana filling turned brown after a day in the fridge?!

You can add a little lemon juice to filling. It will slow down the browning of fruits and vegetables. Yes it is still ok to eat. It has most likely just oxidized like avacados and apples do.

Bananas will go brown by oxidization within as little as an hour after being out of their skin. Unfortunately there is almost nothing you can do about this, short of having a hyperbolic chamber . And yes it's still good to eat just looks ick. when it starts to smell funny don't eat.

yes.

Wrap it with cling film.

thats what my mum did to my banana birthday cake.It lasted for two weeks!