Interesting. Very interesting. With their typical say-nothing-for-sure-explicit manner of conducting diplomacy, they want to further discuss things, leaving their options absolutely open. But at least they say they "think" they understand our proposal. However, it does not sounds enough for them, they want more precise one. The no-settling zone of two rows of tiles I think is enough far away. But then, looking at our proposed no- settling zone, we can even benefit from this their desire. We will say we will step back 1 tile south and if they take back 1 tiles north, then we have whole 4 tiles distances from each border city, so we wont even overlap BFCs and thus no sources of frictions and needless culture wars.
We can settle the tile east of the Marble. We still get nice green land and the Sugar in the no-one's border land. Where, if they step back 1 tile, they will actually have no where to settle except way east from the disputed strip of land. I think they will want the Sugar in their control though. And we might want to agree to that only to have NAP and secure border. If we play our cards right, we can take that city later without risking war with Uciv now when we are vulnerable

Despite all, they ask about more precise city planning instead of telling us what they think is good, so we go ahead and propose them our vision (which will be good for us of course) and they will have to take it in consideration.
They also mention Zulu. We must know why? Do they want not to have common border with Spaniards? Maybe this can be in our favor too. We have no uncertainties about that. We want the land and will guard it. Also it can be nice to not give our 2 neighbors to have common border so they are forced to get in touch more in depth diplomatically and also giving them corridor for moving armies/units between them outside of our control.
As for what contact we have with Zulu, we can say we dont have a NAP despite we wanted and we had some unconventional time negotiating with them, which is the truth. About how they are integrated in to the border agreement, I dont know what we to say. Maybe that we have strange system with them with calling on city spots sequentially and now it is our turn to call a site, so we are communicating border only with Uciv at the moment.
What else I wanted to say about Uciv is that they have 10 cities already. Thats not small count. But their GNP is not impressive as I can see and is nothing like our own. (Yossa?)
This tells me few things. They might be not that stubborn in negotiating about the border are, as they might not want to settle many more cities any soon to not wreck their economy. Another thing is they will be a bit slower on research and getting to maces, knights, cuirrassiers, rifles, cannons, etc. So we might have good window of opportunity to get them with overwhelming technology gap and we might have quite juicy bounty to get with their many cities. Third thing that comes to my mind is that we must track how they settled those 10 cities. Was it periodically settling each few turns once city or it was campaign like settling 3 cities, then pause, then settling another 3 then pause and see if they are even stopped with settling. Also, can we make a guess how they settle their cities? Are they grouped tight to minimize maintenance or they are spread to claim maximum land. If we can get their matrix of settling, we can make good prognosis about their curve of development. Inevitably they having settled many cities and big chunk of land will result in lowering their research capabilities for some time, then their curve will start to grow and at some point all this land will translate in massive amount of research/production for them. But here comes our role to assess and predict this and act before they reach the point where their land will skyrocket their research/production. With more conservative settling we could be able to have smaller fall in our graph before we start to climb again and I hope we will hit higher amounts of research way before them. And the moment in the time where we will have the biggest difference (advantage on our side) in our economics, there we will have the opportunity to strike at them.