I've posted my draw of treaty lines, after the text:So, I got some doubts/need of validation about the Madrid and San Ildefonso treaty lines. Don't know which thread is the best one, this, FleetingRain's one or the tinto maps, but I chose this one. I made proposed lines for the treaties and I wanted to share to validate my choices.
I tried to read the treaties, but it was hard to understand, so I took the description of Braziler as a base.
I refer here to the souther end of the frontier, today in the territory of Rio Grande do Sul.
The treaties worked with the river and watersheds, so as a base I took this map from the Compania de Pesquisa de Recursos Minerais dated of 2005. I then separated the basis I consider important. Ibicui, Jacuí, Negro, Camaquã, Piratini and Jaguarão rivers.
View attachment 1242983
The situation. Watersheds in black, today Piratini and Ibicuí rivers in orange. Ibicuí-mirim and the Santa Maria rivers in pink.
1) Let's start with the treaty of Madrid. It follows the watershed up to the Ibicuí source. The watershed in question is Black and Ibicuí river to the west and Jaguarão, Camaquã and Jacuí rivers to the east of the line. The problem: today the Ibicui is consider the junction of the Ibicuí-mirim and the Santa Maria rivers (point A) and it does not have a proper source. There are many possible sources to follows.
If we follow the Ibicuí-mirim or one of it's tributaries, the source is northeast of point A. Point B is the source of the Ibicuí-mirim, for exemple. This is actually somewhat similar to the drawing of the Mapa Das Cortes that was the official map of the treatyof Madrid(red line in the map). I must mention that this map is awful in this region, so although official, it's not a good geographical source.
If we follow the Santa Maria river, or it's main tributary, the Ibicuí da Armada river, we end up with the river source way to the south. The Santa Maria river source is point C. Ibicuí da Armada river has many tributaries and it's sources are to the west of point C. This is more similar with the map present in the Wikipedia page.
The difference is rather big, so which one to consider? My initial guess was using the Santa Maria river source (C).
2) Now, the San Ildefonso treaty, which was even more confusingly written, but is in accordance to the interpretation of Braziler it is easier to draw. It follows the Mirin lake, then the São Gonzalo channel and up the Piratini river to it's source. Question: which source? It has numerous tributaries. Point D is in the source in the map I took as a base. Other possible sources, arroio basílio and it's tributaries, are a little to the west of point D.
3) Also in the San Ildefonso treaty, after the source of the Piratini river, it follows the watershed between the Jacuí and Uruguay basins up to the source of Garita river. However, the Piratini basin does not touch neither the Jacuí or Uruguay basins, as it is comprised between the Camaquã and Jaguarão(Camaquã goes to the Patos lagoon and Jaguarão to the Mirim lake). To solve this, I just followed the watershed of this basis up until I reached the Uruguay basin.
View attachment 1242984
Proposed lines. Madrid in yellow, San Ildefonso in green.
So, does it seems ok to represent the treaties? Any feedback or ideas?

It's a cropped section of full map, and like the same line of your map to me.
I have given up on representing them in my proposal map because they create some difficulties in the balanced design (in relation to size) of the locations. Furthermore, these lines were never effectively established as a border. But depending on the developers' design philosophy, they can be easily implemented.
By no means belittling your work, but theoretically (not always applied in common denominations) a river is named from its mouth (whether in another river, lake or sea) to its furthest source.
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