We already know the secret Takafumi learned/forgot back then. It was the detail about Uncle being sold for 3 copper when a dirty scouring brush was sold for 120 copper. When we saw flashbacks of Uncle's first days in the other world, Takafumi and Uncle got nosebleeds when they saw Uncle being sold, because relearning something you already forced yourself to forget causes your nose to bleed and your suppressed memories to return. Reread chapter 15 if you want a refresher. I do think it's possible something bad happened to the tsundere elf at the end of his journey and he erased his memory and returned to Earth to escape the loss of someone he loved more than himself (and this may be the reason Uncle is too dense to even retrospectively realize the elf had feelings for him even now), but no, that isn't what Takafumi begged to forget in chapter 1.
So, if uncle is now at the DS, when are they gonna show him sonic adventure 2? It should be less of a shock than seeing that sega game on a nintendo console.
Is the translation a mess? A little bit yeah. Should you be thankful that's it's even translated? Fuck yes! Talk about ungratfull entitled readers. I'm just glad I can read it here (for free).