it's cool to read all your theories, but after watching the first episode again, i think it's something else. i have 2 theories, both can be true at the same time.
I think as some have mentionned: the first scene is already part of a scenario. feng's an actor and when playing in the story he is heng, they start off the same person.
at the end of the series after after both leads realize they changed the script, Heng also is changed. hence he broke through the script and was able to migrate to the same plane as feng. It's the only way we can interepret the portal
However, back to the first episode: the second theory is that feng has been going through a loop as he was oddly playing with a rosary and eating nuts ( which is very historical ). it may be that in that scene it was already heng ( and feng was just a façade just like the 'character' sym and NPC characters since feng himself is scripted) also feng was complaining that he did not look good in the drama and that should have been changed. maybe he was also trying on his side ?
just like he passed a portal in the end, he may have passed a portal in the beginning? the true goal of the show was to prove that Heng character was able to wake up from his script and write his own will.
i also really loved how the 'same' event can be perceived very differently depending on the info you have. and how the writers/ scripts are used as a metaphor for freewill. classic but well done.
what do you think?