No matter how much he hates it, with Jaime gone, Tyrion is Tywin's only heir, so just like in the story when Jaime was captured. Tywin would start to treat Tyrion with a bit more respect and completely shut down Cersei's accusations toward Tyrion in Joffrey's murder, even if he had to go as far as setting up a patsy, which he'd likely do even if he discovered the Tyrell involvement because he needs their support and knows everyone is better off without Joffrey anyway.(Littlefinger/Sansa probably taking the blame since they've switched sides anyway) Tywin would go back to preparing Tyrion for leadership, to take over the rock, just like he did when Jaime was captive, when he made Tyrion hand, while also being more hands-on in molding Tommen to rule. Then his best option for Alliance would be to further solidify the Reach, Margery, and Tommen marriage as quick as possible followed by Tywin returning to Leyton Hightower, now that Tyrion has the prestige as heir to Casterly Rock to overcome his physical shortcoming, along with Leyton's youngest daughter's severely damaged standings. Offering an annulment from the disgraced slaver Jorah Mormont and a Royal Pardon for his daughter to return from exile and become the Lady of Casterly Rock, (Lynesse even sounds like Lioness, which is fitting for the Lady of Casterly Rock, and mother of the heir) in exchange for the backing of the largest force in Westeros, untouched by combat since they packed up and went home after Renly's death without losing any men (especially if Robb/Stannis have taken car of Euron before he goes south to the Shield, then Old town). Lynesse is a Pure Gold Digger, she would jump at the chance to return as the Lady of Casterly Rock, regardless of Tyrion's looks, and Tyrells, Redwynes, and Hightowers all together more than replace the numbers the Lannisters have lost and keep Tywin/The Crown competitive. Even though he will have completely lost any chance for alliance with the Freys or Boltons, since being married into the royal family would keep them firmly in line. Robb marrying Roselyn also puts an end to Ramsay. Regardless of his age concerns, Roose won't legitimize him, especially now that his line could be connected to the Iron Throne and not just the King in the North. He would give Ramsay up to "Hornwood Justice" to protect his trueborn heirs from him.