It certainly seems like an interesting game engine. I do have to ask, is it possible to split the plot into multiple files so that it is easier to follow when writing it
It could be done, but you'd need to change the scripts at least a bit so that when the game is loaded it combines the plots from multiple files into one in-memory dictionary/javascript-object.
You can also just split a big plot line into smaller sub-plots using a jump node (eval_jump_to). From the player's perspective this will look exactly the same, but the editor will not evaluate a jump node and simply end drawing the graph if it encounters a jump node.
I currently have a game with 19 384 nodes (the plot.txt is 3.3MB) and the size of the plot is not a problem. True, some paths are convoluted and terrible to view in the editor, but there's a 'Draw from this node' button that hides pars of the graph that are unreachable from the select node.