I've watched the entire series several times (best way to use your time during a pandemic, imho). Season 3 has Olivia and Fauxlivia switch universes. The idea is, its not easy to cross over, and those that do need to be put in a hyperbaric chamber for weeks to recover. The reason Olivia can is because of the injections she got as a child, gave her special abilities to be able to cross over seemlessly. Fauxlivia did not get those injections, so how was she able to cross without needing the hyperbaric chamber? Recover from the jump? She was able to meld right into Olivias life. Same for when Walter went over to get Peter, why was he fine physically (with the exception of being shot)?
