Re: the Iron Throne - Robb had no interest in staking a claim to all of Westeros. After all, his bannermen had declared him the King in the North (or King of Winter, if you prefer). Robb's interest, beyond the safety of his family, was the sovereignty of his own realm, not expanding his power to encompass as much of the Seven Kingdoms as possible.
Yes, and that was his greatest mistake. If you're not willing to go for the first prize, then you shouldn't play the game at all.