1. Actually, the second dimension is also correspond to the DIF effect. But, the DIF is treated as random effect in the mixed dimensionality approach whereas it is fixed effect in the one dimensional approach.
2. The mixture model could be used to classify the examinees into different latent classes.
3. Yes. It is possible that the speededness is due to the guessing behavior. But still, it means there are two latent classes ( not guessing latent class vs guessing latent class).