1. If Robb had married Walder Frey’s daughter, he would have been the Lord of the crossing after Walder Frey. Tywin Lannister could not have done anything to influence the Freys or the Boltons. Ultimately Robb would have lived! But his love for Talisa made him break his mother’s promise. He would have made a great king as he was a very good war strategist. It was love that killed him, but it was also his foolishness that brought about his death. In the first place, Robb should not have gone to Walder Frey after breaking his promise.
But irrespective of all the events that lead to his death, what would have happened if Robb had fulfilled his promise to Walder and married his daughter? If Robb had married a Frey, he would have marched on to Winterfell to take over the castle from Theon. Now we have to consider Ramsay, the illegitimate son of Roose Bolton whose psychotic ambition would have brought him over to Winterfell. He would have taken over Winterfell from Theon and after Robb returns and he would have handed over the Castle to him as per his father’s wishes. Obviously, when we consider looking into the other Starks, Robb would have been reunited with Arya, who came looking for him. If you can remember it was Arya who saw the head of Greywind pierced on top of the corpse of Robb Stark, as she came along with the Hound looking for Lady Catelyn and her brother.
2. If Robb was married to one of the Frey daughters, he would have taken over Winterfell and would have been united with Arya who comes looking for him. She would not have gone to Braavos and all the incidents that lead her back to Westeros would have to be eliminated.
Since the Starks are at Winterfell, it is very much less likely that LittleFinger would have helped Sansa. Hence, Sansa would have stayed back at King’s Landing and would not have married Ramsay or joined Jon at the Wall.
Bran also would have come back along with Rickon, Summer and Shaggydog. He may not even have discovered that he was a Warg or the Three-eyed Raven. He might not have discovered the real identity of Jon Snow. That secret in the Game of Thrones would have been lost. Plus. Stannis would never have come up North. Even if he did, he would have made a peace alliance with Robb Stark and together they would have attacked King’s Landing. This time, ultimately winning the war as Tyrion has escaped from Westeros and Jaime cannot fight with his left hand, also in losing Tywin Lannister there would not be anyone left to make a good war plan. Robb would have rescued Sansa and Stannis would have been made the Kind of Westeros.
3. How much different the Game of Thrones would have been if Robb Stark was still alive?
Robb Stark was a character with a lot of promises and a carried a lot of powerful expectations among the viewers. If Robb was alive both he and Jon would have joined forces together and made plans to take over the Iron Throne. Jon would not have anything to do with the Throne but he would have done it for Robb. Ultimately all the siblings would have been united at Winterfell, given that Arya finds Robb, Robb rescues Sansa and Bran and Rickon return to Winterfell.
It still pains me to wonder how George Martin could out of the blue kill a character such as Robb. But with Robb in the picture, we could only see him growing while his other siblings still had no identity for themselves.