If I sign up can I download a bunch of lists and continue practicing if my account time runs out and closes? Or have they set it up so you need to always check in?
I have Pleco and it's the best app I've ever used. You just pay up front and there's no gimmick.

You can always review things that have already been added into your study queue, even if you no longer have a subscription.

If you add a list and all of the sections have been added into your study queue, then you'll be able to continue studying that list even after your account expires.

Can I add a whole textbook? And if so, what's the point of paying month to month?

I think that maybe you can only add a hundred words at a time. In any case, it is not very pleasant to add too many words at once. It's like drinking from a fire hose.

If you were able to add a whole textbook at once, the scheduling would become hectic and not very efficient. It's recommended to add new words progressively as you are able to handle them, otherwise it'll be a mess. If you are already very proficient in the target language, it might not be as crazy, but is still an unwise idea.

I like the analogy of drinking from a fire hose, that's about right! ^^