You can't return past the force arrows in Part 4 - 4N.
Last Level - 1N is also impossible because the green or blue doors can't be opened. Perhaps a mimic potion behind a red door could do the trick.
Few other things:
2000BC - Entrance:
You don't need the goblin behind the force arrow to kill the second serpent because you can use the first goblin.
The Cliffs - 2S:
Was there supposed to be some kind of trick in this room? Because you don't have to worry about the other invisibility potions at all. If you replaced the crumby wall with a north-pointing force arrow and added a green door to the southern area of the room, that would be a different story...
Last Level - Entrance:
I tried to use the goblin but the serpent killed itself in a dead end before that