I can confirm the email to be true: GM is planning on opening assembly plants back up starting May 18th. They will begin with trucks, then SUV's and Corvettes. That is to say, Corvette production will likely not start on the 18th, but perhaps a week or two after.
They will begin with 1 shift, then slowly ramp up to two, then three. Employees will need to be re-trained on everything as they've been away from work for so long.
Edit to add: The situation is fluid. If things take a turn for the worse again, the plants may not open or may be forced to re-close.