b) Sun moves.

In your flat earth animation, there is no reason why the moving sun would not illuminate the whole 'flat earth' all the time...
We can see the whole flat earth, therefore the light from the 'sun' would impinge on all the earth all the time...
You make an arbitrary cut-off for the light with no reason.
Also, what 'force' causes the 'sun' to move in the manner you show...
Occam's Razor, my friend ...