As pointed out previously, the main idea for more than half of the techniques you can execute from 50/50 is to get your trapped (right) leg across to their opposite hip. This is easier said than done, so start by creating some distance between your partner's upper body and yours. A left-side stiff-arm does the trick. Next, hip out so that your right knee goes to the ground. If necessary, you can use your free right hand to help open your partner's knees up (you don't need much space here).

Now, keeping your right instep flush with your partner's left hip, hip out to move past their right knee (this is now going to be the main way they block you from going around to their back). Essentially, you're trying to create an arm-drag position, with your right hand on your partner's far collar, and your left hand on their sleeve. Just drag the arm across, take a temporary double underhooks back control, and secure your position as normal.