Flat Earther's... If the Sun is setting in Australia then how can it be also rising in The America? explain the physics and math?

I think you'll find that your typical flat earther left school at 16 after struggling with arithmetic but excelling in quoting the bible. So you're going to have a long wait to get any logical explanation for their bizarre conspiratorial belief's.

Easy. We actually have 2 suns, like Tatooinee in Star Wars.

Another  question is  why do the   stars in the  northern hemisphere   rotate  around the north star  while the stars in the  southern hemisphere  rotate around the  southern cross?

Not a flat-earther, but if there is to be a solution, it would have to be: there is no bottom of the world. I.e. Imagine the unit circle. After traveling 2pi you're back at 0 degrees. I.e. half a circle would behave as a full circle.

The only other solution I can think of is that the placement of the continents would have to be brought into question (after all, if you don't believe the earth is spherical, why would you necessarily believe the modern view on the placement of the continents?)

there are no 'flat earther's' it's just an insult thrown at people by idiots who disagree with others.