What do you think Renly was about to do then, once he got to Kings Landing? Think he would send Joffrey on his merry little way to Casterly Rock? Nope, he would have to kill him or have him locked up forever ( at best ) in order to be King. By conquest or otherwise. Stannis would do the same, true. However, he was right in that Cerseis children were illegitimate while someone like Renly simply didn't even care, and thus as far as the Baratheon dynasty goes, Stannis is the rightful King. But then, all this talk of legitimacy to the throne is flimsy to begin with.
As for fair battle? What do you think Daenerys means to do once she gets to Westeros with three Dragons? Give every honorable knight the fair battle they are due? Hell no, she would have them bend the knee without a fair battle or she would give them another Field of Fire, and instead of killing one man, the leader, would probably see thousands of people die burning.