Yes, submitting for official critique does indeed use up credits in your account - so if you have 1 credit and submit your Lesson 1 work for official critique, you will have no credits in your account afterwards until you are next charged by Patreon.

As to your second question, the answer is also yes - as described here in Lesson 0 the system is designed so that those who have tighter budgets can sign up for the Patreon when they need credits, and pause/cancel in between, while those with more flexibility in their budgets can maintain their pledges and allow some credits to expire. As we pay our TAs more than the base price to receive feedback for the critiques they do in order to ensure they are fairly compensated for their work, those credits that are allowed to expire help cover the costs of those who need to ensure every one of their credits gets spent.