Can my company cancel my work permit and extension of stay if I leave Thailand for a month and don’t return?

Your company can cancel your work permit, but they cannot cancel your extension of stay. That's in your passport that will be with you, and even though it will be voided if your work permit is cancelled, that's not the same thing as you going to the immigration office to cancel it. Until it's cancelled you cannot get another long term extension in Thailand, and you might be fined 20,000 baht for failing to cancel it in a timely manner.