For Frederick to inherit George II and his offspring would need to be dead, unless of course George II daughter oldest daughter is alive and have married Frederick. But I can't see the British choosing the Hohenzollern if they can marry their heir to a foreign monarch. I would say the Danish would be more likely, as it would strengthen alliance network (which Prussia was already part of).
Pretty much this. If you want Frederick as king in his own right then the entire royal family needs to die. But if Amelia survives then she becomes queen herself and her husband becomes King Consort/Prince Consort beside her.
For the British to not protest overly much she'd need to be married off before the royal family's death. Perhaps after Frederick's escape attempt he gets exiled to Great Britain for a time and a royal match is hatched by Sophia Dorothea of Hannover and Caroline of Ansbach. Though von Katte would need to live, if in exile, if you want Frederick in a healthier mindset at this time.