Yes, pilot episodes are there to test response and gain awareness.
Nope.
They may leave the story open-ended or give hints toward the future but I can assure you they do not write a full season of episode scripts before they can see the response to the pilot. At the most maybe one or two episodes, but 20? Ridiculous. Let’s look at Ninjago, which started as a piolt mini series. Their initial run completed an arc, and left some open ends. The standard pilot-test strategy. The show hit well with their intended audience, so more episodes were commissioned. However they didn’t start the series again until nearly a year later to promote the next year of toys.
Now I’m not sure how Lego is going to pace the episodes compared to the story. They could try to do the entire year in 4 episodes or perhaps 4 for the winter and later 4 for the summer. At this point though I don’t expect a fully fledged show until next year if JTO performs well.