Yes they would use your 2019 if your 2020 is not filed. They also process the stimulus payments in batches, so let's say they start sending out payments April 5 and you filed April 14. That doesn't guarantee that they would use your 2019 taxes. There could still be a lag from the time they got to yours and your 2020 taxes could be processed before that. Hopefully they start sooner than that.
