Hard to say at this point. I am predicting somewhere between 1.5b-1.8b. 2b is almost impossible going by current pre sales numbers.
Not to say it will make 2 billion, but isn't the opening weekend projected to be The Force Awakens-level? If that movie topped 2 billion, then why would it be so impossible for Infinity War to do so assuming it opens similarly? (not to say it will make 2b, just responding to your point about the pre sales).
It cant. If it does do a Force Awakens level OW then it will become too frontloaded and drop massively like Harry Potter and deathly hallows part 2, Civil War, BvS etc. Force Awakens had no competition in December, December legs (People from 9-5 jobs and schools get time off) and way more hype.