Basically, I think it was fair. If no-one else had their aircraft ready that was their fault. Moreover, with more development time the Macchi Mc72 could be expected to be faster. Given the same amount of time to develop the S6, the playing field would have been level again, but Supermarine had played by the rules and got their aircraft finished in time. Sure, it would have been more sporting to decline to compete. or agree to the postponement the Italians asked for, but how can it be unsporting to abide by the letter of the rules? It was a competition after all.

On your second question, I think Mitchell would have looked at the cantilever wing again. This had caused the S4 to crash and the S5 and S6 used a braced wing, but with the type 224 and later the type 300 (Spitfire) Mitchell went back to the unbraced cantilever monoplane wing – in the latter with some success. So I think this is probably where he would have taken the design.