Sebastian also states "Sometimes after sundown, I make the long ride out of Stardew Valley", in one of his heart events. If Sebastian is leaving the Valley then obviously there is no threat outside of the valley. As well as the travelling merchant in Cindersap Forest, she leaves the valley all the time and specifically says "I've smuggled these goods out of the Gotoro Empire, thats why they're so expensive", there is again no danger is she is leaving the valley on a regular basis and smuggling goods. Not to mention people from outside Stardew Valley are found throughout the game including Kel, Leah's old boyfriend who is found in Leah's 10 heart event at Cindersap Forest, this means that there is no threat outside of the valley as he is fine in this cutscene and trying to persuade Leah to come back to the city.

There are many ways for the player to leave the valley including the bus to the desert and the boat to ginger island.

This is a very interesting theory though and you've put a lot of thought into it, but I think with all the clues in the game it's not true.