The intention is that you submit all the assignments of a lesson at once, yes. I can understand your concern about wanting to make sure you're doing it correctly as you're doing it, but you really don't need to worry -- you will have plenty of time to incorporate feedback and improve at each exercise once they enter your pool of daily warmups (which happens to all the exercises of a lesson after you complete it).

You can do partial lesson submissions, but I'm honestly not sure if anyone looks at those. Again, I'd recommend just uploading it all in one fell swoop.

Part of the reason for the cooldowns is to encourage you to stick to the 50% rule, but also, I think, to give you ample time to practice these exercises in your daily warmups. If you were to blast through all seven lessons and challenges in a month (I don't know how you would physically do this but I'm sure someone's been crazy enough to try), you would experience all the content of Drawabox, but you likely wouldn't have the same level of draftsmanship skills or the understanding of 3D space that the student who took many months to complete all the lessons and was diligent with their warmups has. Unless you're already a very seasoned artist, of course. It takes time and many good nights of sleep to process all this new information.