There are really two ways for Roger of Mortimer to survive 1330:
1. Edward III deposes Mortimer, but leaves him alive.
2. He maintains his position as royal favorite, he continues to reign as de facto first minister of Edward III. He probably continues his relationship with Isabella, and continues to increase his own fortune, while alienating more barons and knights.
Option #1 is less likely because Edward III probably recalls that Mortimer’s last exile ended with him returning at the head of an invading army. Simply dismissing him from court could also be dangerous, given the large amount of wealth and land that Mortimer had accumulated since his victory over Edward’s father. There was also the fact that Roger was a living embarrassment to the King, the man who had deposed and murdered his father, who had dishonored his mother.
Option #2 would not be a sustainable solution, given the character and temper of Edward III. Edward III had seen firsthand the consequences of a passive kingship from his father, and was not inclined to allow his realm to be ruled by a royal favorite. For Roger to continue to dominate the court, he would have to keep Edward II as a figurehead, while maintaining the confidence of Edward’s mother. Ruling through intrigue would be Roger’s only real option; he was not a leader in the mold of Simon de Montfort.
As long as Roger remains alive, relations with the French are likely to remain icy, but not hostile. He would be interested in maintain the English foothold in Aquitaine on behalf of his nominal lord, but would hardly be in a position to prosecute a war. One of the more interesting possibilities would be for Roger to encourage Edward’s abortive efforts to organize a new crusade with his cousin Philip of France. If that plan actually came to fruition, Roger would have at least a few years to “look after” the Kingdom while Edward went off with the French King to battle the Mamluks.