If there is any possible way we can turn Stannis around to our side, we shall not hesitate to do it. I promise. [He really doesn't see that happening though. All he knows of the elder Baratheon are stories of how unmovable and unrelenting he is. If he believes the throne is really his, he won't give it up easily] He may not be a king but perhaps he would make a decent Council member. And if he so desires Storm's End, you could give it to him. You won't need it, you'll have King's Landing.

If we manage to ally with the Starks as well, the Lannisters cannot hope to win.

We can hope. [He doesn't see that happening either, though]