While deaths are mounting rapidly in the U.S., the nation’s second surge in coronavirus cases appears to be leveling off. But experts say the flicker of good news is driven by changes in a few large states and that cases are rising in many others. Indeed, few scientists are celebrating. Some are skeptical that improvements in states like Florida and Texas will endure. They are watching worrisome upticks across much of the country and don’t know when deaths will start coming down.