Logically, this does mean that since your warmup pool is empty prior to completing Lesson 1 (and confirming that you understand how to approach those exercises), you won't have any warmups to do. So don't worry about the warmups until you've gotten through the first lesson and received that confirmation.

Note: Keep in mind that everything we recommend or instruct applies only to the time you're spending working on this course and its homework (which does not include the 50% rule, which pertains more to how you spend time outside of Drawabox). So the "sittings" we refer to are when you're working on your Drawabox homework specifically.

https://drawabox.com/lesson/0/3/warmups

It depends. You are not using official critique so you are not obligated to follow the rules in the same way if you were doing the paid course. You could just move on to the 250 box challenge if you wanted to. Community feedback is not guaranteed, so it's possible you won't get any at all. You are more likely to receive feedback by requesting it in the critique exchange channel in the Drawabox discord.