Efforts to overhaul the healthcare system have moved ahead rapidly, with the insurance industry making several major concessions and the chairmen of five Congressional committees reaching a consensus on the main ingredients of legislation. The chairmen agree that everyone must carry insurance and that employers should be required to help pay for it. They also agree that the government should offer a public health insurance plan as an alternative to private insurance. But members of Congress are just now turning to the most explosive issues, which could delay or derail the process.