1. the inexplicable ability for a phone-microwave to rewind time for the entire world (not just inhabitants of a time machine, as appears later).
2. This ability for one person (or a group of people near the phone-microwave) to remember other world-lines, but apparently NOT the future history of the world line they are currently now within.
2. SERN developing said technology and yet not resetting the world to gain what they desire (or doing so in another world line dimension that MC is now unaware of).
3. Changing the past is moving a world-line, so time travel to your double past is impossible as you then create a new world-line. John Titor travels to the past twice, and yet reaps the benefits of both time travels in a single world line, impossible per world-line mechanic (travel creates new multi-verse). The paradox is that you would have to already exist within the world line where time travel to the 70's happened before time travel existed to travel into the 70's, and thus time travel to the 2000's reaping those later leaps.
The plot-hole could be fixed if time travel to the 70's happened FIRST, then forward time travel to the 2000's to hang with parents happened afterwards, but this diverges from the IRL John Titor plotline the story follows, so it adopts the plot.
Even when factoring out paradoxes by the presupposition that everything that can happen does happen, we're left with a story that can only exist within a paradoxical set of world lines that can't retroactively influence one another.