(G a)⊕(F !a) holds on the structure, and ((X G a)⊕(G !a)) does not hold on the structure, so yes, the first example is an example structure that distinguishes between the two formulas.

In the second example, I am not sure what formulas you exactly mean, but despite of that, you are also right, since our teaching tool tells us the following:

- (G b) -> F[a WU b] is valid
- (G b) -> [(F a) WU b] is valid
- (G b) -> ![(G b) SU a] is not valid
- (G b) -> !G[b SU a] is not valid