UA is obviously outrageous, not counterable and quite unexcepted. Oh, I would not send a Trade Route to them. I would rather wipe them out of game. I believe they have decent flavour making them decent target of trade routes. Though, I would probably change it to "leech gold from Cargo Ships crossing your territory, which owner has no naval Trade Route to you, automatically destroy if at war (+200 gold)". I believe you can make it (we can't connect unit to trade route, but we can read domain type). Simpler, easier to balance (large maps, 18 * Era gold per turn is huge, I learned it once) and more reason to settle strategically points... (my little "game in game").
Well, my thinking for the UA was not even to get more people to trade with you (which wouldn't work that great outside of MP, and Morocco does the same thing better, anyway). I actively wanted to give them a "bonus" that was a negative for everyone else rather than a plus for themselves. I know people probably outright hate this. It's somewhat like the Great Wall (which is probably a bad example to bring up because I know people universally revile the Great Wall). The Great Wall doesn't really give you a benefit per se, it just gives a negative to everyone else. It was a direction no other civ in the game took. (It also struck a historical chord as Khmer, Vietnam, China were constantly complaining to the Cham kings about coastal raiding populations which the Cham monarchy had little control over).
As for the implementation. Early game, it doesn't do anything. Mid game, honestly, 2-3-4 gold per turn is not enough to make me go out of my way to destroy a civ on another continent if I wasn't planning on doing so already. That's the reason for the trade route exception. If you are on the same continent as them, you're probably trading with them anyway, so you don't have more reason to kill Champa if they're your neighbor. Late game, well you're trying to kill/win against everyone anyway late game. Plus, if every civ has a negative, it's impact on you is much more mild than if only you had it. The leeching gold from someone's treasury when you're are war aspect I actually really like (even if I remove the first part, I probably wouldn't remove that). I doubt any of the values for any of the abilities are balanced as I didn't test them they way I would if this was a final release.
And actually, when I was designing the civ more as a pro no trade route civ. I did have almost the exact ability you suggested coded. I dropped the no trade route direction because, well, you ended up doing a lot of work just the get the same benefit everyone else got from trade routes. I then limited the stuff from foreign cargo ships just to the UI because I didn't know how effective the "game in game" would really play out. I convinced myself that as much as I liked the gameplay idea, its something that in actuality would probably be overly tedious and annoying to play. So I wanted to see how the UI actually played out before deciding on expanding the design back to other areas (for this or future civs).
About the UU, keep in mind that the unit can now also attack other units without setting up. At the same cost as a crossbowman, the elephant can double as somewhat weaker crossbowmen in addition to attacking cities. In terms of strength vs. cities, the unit maintains more damage vs a city than the standard trebuchet up until the third turn (say, 4-8-12 instead of 0-6-12).
By the way, if people universally hate the "negative UA" design I will probably change it (or more likely release two versions of the civ with two different UAs) once I have art.
As for the dependency, lack of original improvement art, (though for someone of your modding skill it's no trouble to edit the Art Define of the improvement in the XML to say the Moai and erase the dependency line in the modinfo file).
). There is a drawback in some scenarios.
