As mentioned in Gil day 7 and shown in Episode Anders, Herbert was Anders' classmate in elementary school in Highwell while Walter was homeschooled, and they met again in high school. He appears in Episode Anders. Edgar asking Walter if he wanted to talk to Herbert wouldn't make sense if he was dead.
The twin switch idea is weird. Yeah in Parker's route there's that dream, and Gil thinks Herbert got sick more often when Walter remembers the opposite. But that seems to contradict Parker having fond memories of himself Gil and Lars taking care of one of the kids in the friend group who got sick often, which can't be Herbert since Gil only met him once or twice. And Walter having a dream memory about the rice ball story in Parker's route also goes against the switch idea to me. If they switched, he wouldn't have the real Walter's memories. Lars and Parker both say Walter hasn't changed at all, and Herbert is said and shown to have an opposite personality. If they switched, would he really be able to act exactly like Walter?