Can I start by saying that I absolutely adore this game - what you have made here is a work of art and I have thoroughly enjoyed playing it. I might just be missing something very obvious, but I can't find the final material that I need to plant in order to (I assume) finish the game. There are no obvious places to find it, and no new conversations to trigger, so I am at a loss as to what to do next.
If you're stuck, I'd recommend double checking that characters don't have anything new to say on repeated conversations in other branches.
If you're really stuck, here's a full spoiler answer to where you might have missed things (in rot13):
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