If you buy one now you should be able to claim any credits available for the quarter you purchase the car in. The IRS master list is a guide only - it doesn't show the 2016 Volt either but if you find one of those still on a dealer lot it would also qualify. CARB is a different story so you'll have to check with them.Check again on Jan 1, 2018 for federal credits. As for CA, I am sure it will be added around the time GM starts shipping 2018 Volts, if they continue to qualify.
GM and Tesla will both reach 200K early next year, probably in Q1 certainly by Q2.The federal tax credit is based on the number of vehicles sold by a manufacturer. The model year isn't relevant. After the limit is reached the full credit is still available for a quarter, after which there is a declining amount of credit available. AFAIK GM has not hit the 200K limit.