[mpg_numbers_guy]

My mom used to have a 2nd gen Prius like yours before it was totaled by a deer..

Grille block helped the car warm up faster in the winter.

Stock wheels are already light weight and optimized for aero, smooth wheel covers won't do any good, especially if they stick out any further from the wheel itself. In fact, they might actually hurt aero. Best bet is to keep the stock wheels with all 4 of the rings in tact.

If you really do want smooth wheel covers, secure them by drilling into the aero rings around the wheels, not the wheels themselves. You may notice a slight drop in mpg with adding the wheel covers due to more weight and poorer aero.

If you delete the mirrors I would definitely recommend rearward cameras if possible, but definitely add a panoramic rearview mirror.

Rear is already optimized for aero unless you add a full boattail, modifying the spoiler will likely result in a decrease in fuel economy.

Tire pressure is the single best thing you can do to help with fuel economy with the Prius, it's pretty much already near perfect in optimization for what it is.