Does rbf require your input to be used with a smaller output?
There should be the same inputs and outputs with no change to the amount of coins sent + a new input which will be spent on the increased fee and an extra output if there are any coins left from the new input. However,
there are different variants of RBF.
But by default, most wallet uses Opt-in RBF
Has anybody tried to send a zero fee transaction? I might have a go if nobody has attempted it.
I don't see the point of attempt broadcast transaction with 0 fees since we know most full nodes & server for SPV wallet won't relay it, unless you managed to connect to nodes owned by pool which accept transaction with 0 fees.
But there's no harm doing that, so you can try it even though we can guess the result.