I've played with Mercantile now that I understand what it does and I must say:
Mercantile seems incredibly overpowered. I tried to exploit it as much as possible and was absolutely crushing my enemies on Monarch difficulty. It felt like normal BTS on Settler difficulty. I played as Dido of Carthage - built the Great Lighthouse (GL), then teched to Currency, then set up Cothons in every city, then Education/Unis, and of course switch to Barter Economy asap.
It's amazing what you can do with Mercantile. The GL gives you 3 (1+2) trade routes per city, Currency adds another one which makes 4 for any Mercantile leader with the possibility of using Castles for a 5th one. Carthage even gets one more with their improved harbours.
Build the GL before training a settler or shortly after your first settler. Then you'll hit Writing and get open borders. Every new city will start with 3 trade routes, 4 after your Currency-beeline. You'll have +2/+2 for Mercantile almost right from the start. All your cities will have at least +4/+4 as soon as you reach early mid game (i.e. when reaching Feudalism). Soon you'll outtech, outgrow and outproduce everyone else. The most powerful thing is that you can build new cities everywhere - along the coast - and they will immediately grow fast, produce monuments fast and provide a considerable tech boost. It's a no brainer.
I like the trait, it's a cool idea. But imho, it needs to be nerfed a bit. Mercantile on its own seems about right, but Mercantile + GL is too good, not to speak of Mercantile + GL + Cothon + Philosophical.
On the other hand Diplomatic seems a bit underpowered. I havent actually tried to exploit it as I did with Mercantile, so I might be missing something. Anyway, how about adding a +1 to diplomatic relations bonus to Diplomatic?
Next I'm going to do a pyramid strategy. With the new means of getting more food faster, this seems exploitable, too.
I hope the critique of Mercantile doesn't come across too negative. I was just blown away by how good it performed.