HONG KONG, June 5 (Reuters Breakingviews) - Shein is threading the world’s trickiest geopolitical needle. The fast fashion phenom, now worth $66 billion, is a formidable rival to Inditex's (ITX.MC) Zara and H&M (HMb.ST) thanks to its marketing prowess and efficient Chinese supply chains – and despite the U.S.-China trade war. But rising American pressure is forcing it to tweak its business model right as it tries to list there. Shein, pronounced "shee-in", leapt to success by peddling $5 crop tops and $15 bikinis on its app - so affordable that some less environmentally conscious customers have claimed to wear...