You asked three questions here, answers to none of which would impact the others.
Atheism can be true or false and that wouldn’t affect the status of God as imaginary or not. God can be imaginary or not and that wouldn’t affect whether we’re here by purpose or not.
Look, a singular God is just one way to imagine our cosmos being created. What if our cosmos were created by a class of 3 year olds in another universe as a science project?
That means that atheism is true, God is imaginary, and that no, we’re not here by accident.
What if God were real and not imaginary, but he had no real hand in creating the universe, that it kind of evolved on its own? Atheism would not be true, God would be real, but we would be here by accident.
What if God existed at some point in time, but died leaving the universe to evolve on its own? Atheism would be true, God would not be imaginary, and we’d be here by accident.
You can keep imagining scenarios to fit any set of criteria you want. The real problem here is the fuzziness of language. Things being true often don’t carry the connotations we want them to have.