Great explanation, and your understanding of the prodrugs is right on target.
However, I think there's a flaw in your logic. Because of the mechanism of action of TFV (it inibits reverse transcriptase, the enzyme that translates the HIV RNA into DNA, which takes place *after* the virus has entered the cell and its RNA has started making proteins), it's only going to be effective after it is already in the cell that the HIV virus is infecting. So it really has to be in there already, which means that having TFV floating around in the blood *outside* the cell isn't going to help prevent infection. Just mess up your kidneys. This would be why Gilead developed TAF. I believe this is also the reason that it's necessary to build up the body level of either drug before exposure - it takes time for it to be absorbed and get into the cells where it is needed.
Infectious disease isn't my field either, but biochemistry is, or was once, with substantial studies in molecular bio as well.