Last Wednesday, the Trump administration believed it had a plan to save TikTok. ByteDance, TikTok’s Chinese owner, along with some of its U.S. investors, and officials in Washington had coalesced around a new ownership structure for the popular video app, four people familiar with the situation said. That structure, the people said, would help TikTok satisfy the terms of a federal law that required the app to find a new owner in order to address national security concerns, or face a ban in the United States. Under the plan, new investors would own 50 percent of a new American TikTok...