It is true, after a round starts you can not reverse trades.
Trading Danger -> Martin and then Martin -> Danger is not reversing trades.
Reversing trades re-sets you team to last weeks ending status, thats why the 'Reverse Trades' does not work after the round has started.
I beleive is is a 'loopohole' that doing the Danger -> Martin and Martin -> Danger costs no trades, like wise Danger -> Martin and then Martin - >Mitchell only costs one trade.
But it only works if you have a trade un-used for the round.
If you did Rowell -> Mitchell and then Danger -> Martin you would have no trades left fo the week so you would be unable to do the reverse Martin -> Danger trades.