If you provide a reference/context to where you got that image, it would be helpful. But my guess is:
the QP is there to maximize excitation efficiency. Excitation efficiency depends on the relative orientation of the electric field (polarization) and the absorption transition dipole of the fluorophore (no excitation will occur if they are orthogonal). So, in theory, in a fixed sample, if the fluorophore isn’t tumbling freely, you could get “unlucky” with orientation and not get excitation. This is not uncommon in single-molecule applications (but I’m not sure how critical it is?)
i suspect the lenses are just relay lenses, (but we’d need to see the paper to tell you for sure)
yes, in all TIRF-like applications, you need to focus the laser spot on the back focal plane… but again, need to see the paper to tell you what that lens is doing