The 'it' in 'force me down it' refers to the tributary. Put in context, it would mean 'someone is trying to force me down the tributary'.

The sentence is incorrect, so it's understandable that you don't understand. I'm assuming they mean 'You're fascinating in a low-key kind of way'. If this is the case, low-key is a more informal way to say 'subtle'. In this case, being 'low-key fascinating', means that they are subtly fascinating, or fascinating in a way that's not obvious. 'Kind of way', is just added to the statement to emphasise the 'low-key'. It doesn't really have any meaning on its own. Saying 'You're low-key fascinating', would still mean the same thing.

High-stakes is usually used to describe something that has a significant negative impact if lost. For example, a bet with high-stakes could mean that both sides are betting a large amount of money. If they win, they gain a lot, but if not, they could lose a lot too. Likewise, IELTS as a high-stakes English test refers to the fact that you could gain a lot if you pass, but lose a lot if you don't.

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