I can't answer a number of your questions, but there are many notable people who have received an LLM in the United States before entering Chambers in the UK. Here are two: Rodney Dixon, Temple Garden Chambers (LLB in South Africa, LLM at Duke in the US). Amal Clooney (Jurisprudence at Oxford, LLM at NYU). My son is planning this same approach - LLB in the UK, LLM in the United States. This is most common for International Tax but it's essential for anyone who wants to also be admitted into the US (one can sit the Bar Exam in New York with just an LLB but one is more prepared after a US LLM (9 months)).