OP: Does atheism in Buddhism mean you let a belief arise that God doesn't
exist? Is this a skillful assumption? Why or why not?
To put your "salvation" or your liberation from suffering into someone else's hands, is unskillful in Buddhism.
To reinforce belief in a self, is unskillful in Buddhism.
To take anything that is impermanent as permanent, is unskillful in Buddhism.
Faith in an eternal permanent Supreme God who is one's personal saviour would include all three unskillful views above, in some way or the other.
OP: Was it right view for one to believe that the world wasn't round
during the time when it seemed to everyone that the Earth was flat?
Was a belief in a flat Earth a skillful assumption? Why or why not?
Whether the Earth is round or flat is not of concern to Buddhism, and can be left to science. In this case, it is skillful to rely on scientific methods.
However, metaphysical investigations that cannot be proven by science is not skillful, as seen in the Acintita Sutta:
"Conjecture about [the origin, etc., of] the world is an
unconjecturable that is not to be conjectured about, that would bring
madness & vexation to anyone who conjectured about it.