Yes it's a risk.
What about selling to Siam in return for war on Nappy? This guarantees no open borders, and gives Nappy something else to think about.
I've looked and Siam wants a load of luxes which will make us unhappy. Orleans is a puppet however with quite a big population. Can anyone work out how many happy faces we get for offloading Orleans?
If we go less than 4 unhappy, then we will get happy with the Taj.
I like the idea of keeping Siam strong as they will be a source of tech for our spies and for tech agreements.
Just an idea.
If we have open borders with Siam though - our units won't move, and they'll all die. So scrub that idea.
Another edit.
Siam had not opened it's borders to us. Our troops will be automoved. Siam has a shed load of city state allies too. That has to hurt Nappy also.
Nappy will also take Orleans from Siam - so what does that mean? Is that good or bad for us?
My rough calcs say that we go 2 unhappy if we go this route.
It's very expensive, but certain. I'm not sure if it's a good idea myself.