You can use the CTL model check to answer your question: With input
E[E F a WU b];
b ∨ (E F a) ∧ (E X b) ∨ E F(b ∧ E F a)
You will see that the state of your structure satisfies EF a, and therefore also E[E F a WU b] (note that b never becomes true, so that the weak until demands that EG EF a holds which is true).
In contrast, neither b nor (EF a) ∧ (EX b) nor EF(b ∧ EF a) holds on that path.
So your structure distinguishes correctly between the two formulas.