It’s seems the main point of contention is not wether the other two had a right to make a mod about the Seatower, but if they had a right to use the art created by Cuv and Yovaneth. The answer to that is, they don’t.
Speaking as someone that actually had to study collage law in school, they could claim the work as an original collage piece of art. Since it looks like their was some photoshopping going on.
To take it a step further, Beamdog states on their forum “everything created with Infinity Engine resources is more or less proprietary to the owners of that engine, mods are the property of their respective authors”. They also say “Using someone else’s work in your own work without their permission is stealing” and “Developers follow these same rules; we don’t use a mod or a resource without first obtaining permission from its creator”.
The other two are not using original game assets, they are using the original collage artwork and mod resources of others without permission. Per Beamdog, that is stealing.
https://forums.beamdog.com/discussion/19418/a-quick-note-about-copyright
For the record, I'm really looking forward to this mod. Amazing video BTW.