Couple questions on South American border disputes:
The maps in the books and here show Bolivia still has the Gran Chaco region. In your head cannon, did Bolivia and Paraguay still go to war over this region ITTL? If not, how was it averted?
I have thought about this issue a lot. The different maps out there regarding the borders and countries for the world of TL-191 don't make it easy to determine though, especially since we don't really get a lot of information on these countries. Some maps do show Bolivia clearly getting the Chaco region, while others have Paraguay getting the Chaco or at least half of it (hard to tell with some of the qualities of the maps).
Either way, I do believe that the issue over the Chaco Region will not go away for these two countries. Both of them are going to want this region and both of them will have to settle the dispute one way or another - through diplomacy or war. Even with the changes in this timeline post Great War, I personally don't see the issue over the Chaco just being butterflied away. Bolivia and Paraguay are both landlocked countries in South America as well as being two of its poorer nations. Bolivia has been yearning for any kind of access to the sea - formally owning the Chaco region gives the country access one of South America's major rivers (The Paraguay River) that does lead to the sea, and thus more effective trade with the outside world and a route to economic prosperity. Paraguay, having lost much of its land and people through devastating wars, would be adamant on holding the Chaco region for itself, to preserve its national integrity, to hold land that would give it more effective control of the Paraguay River, and thus a means to gain trade with the outside world. The potential to exploit the discovery of oil in the Chaco is also something that would drive these two countries to claim the Chaco.
Another reason why I think the issue over the Chaco will still be around is that the power dynamics in South America have changed with regards to some of its more powerful countries - Chile, Argentina, and Brazil. Chile, Paraguay, and Brazil all went to war on the side of the Central Powers in the the Great War and won. Chile and Paraguay in particular fought a land war with Argentina. Post-war, the rivalry and animosity between Chile and Argentina is still very alive. Any border dispute between its neighbors is a potential opportunity to gain influence in the region and gain allies in for any coming war. So as Bolivia and Paraguay press their claims over the Chaco, Chile and Argentina might step in to support any one of these nations.
Another reason why I think the Chaco would still be a problem is because the dispute was already a long running issue between the two countries. Clashes and disputes over the Chaco dated as far back as the 1880s and skirmishes between Bolivian and Paraguayan patrols in the area were already threatening to flare up into a full blown war by the 1920s in our timeline. The reason it didn't was because both countries felt they did not have sufficient weapons or a sufficient army in the 20s to go to war with.
In this timeline, with no League of Nations and the USA focusing inward to deal with losses from the Great War and deal with the Great Depression, along with contending with a resurgent Confederacy under Featherston, international calls from the wider world to cease hostilities may be small to non-existent... unless countries from Europe want to really get involved, which will be hard to guess. Personally, I see this potential dispute being handled primarily by the South American nations themselves. Paraguay, having taken part in the Great War coming out victorious, still might have to content with a very unstable political environment on top of carefully preserving its small population that has surely experienced losses from its war with Argentina. Provided that events in this timeline don't change much for Bolivia, it is likely the country will still want to claim the Chaco.
How this dispute gets resolved is a bit tricky though and could happen a number of ways. Turtledove not mentioning the Chaco dispute may be down to him just overlooking the thing entirely and forgetting to write something for it or that a war never happened - I lean more on the side that says the dispute was occurring and that war was possible.
One way that it might get resolved however is through peaceful negotiation. In this case Brazil might act as a neutral arbiter rather than Argentina (given that Argentina is more clearly aligned with the Entente, though Brazil being neutral arbiter isn't exactly ideal). The Chaco might be partitioned equally between the two nations, which gives both of them equal access to the Paraguay river. Clear navigation rights on the river for Bolivia would have the be hammered out to guarantee that both nations may use the river equally.
Another way this gets resolved is, of course, through war. When this war will happen might depend on a few factors. One of the reasons each country did go to war was that both felt they were ready wage war once they had built up their weapons stockpiles. Paraguay in this timeline may have an advantage here - taking part in the Great War means that their army has already gained experience in battle and they may already have fairly sizable stockpile of weapons built up thanks to potential support from the Central Powers - getting guns from the US, Germany, and Brazil. If the skirmishes in the Chaco in the 1920s flare up into a war, Paraguay may feel more confident in its ability to wage war compared to Bolivia. Pressing their initiative and chances the Paraguayans gain the advantage in the skirmishes in the Chaco and push out Bolivian patrols more effectively, thus giving them better control of the region. These skirmishes might well flare up into a war, but perhaps Bolivia might be more pragmatic to back down after failing to gain progress. The Chaco War may occur, but might also be a far shorter, far less bloody affair that goes in Paraguay's favor. Chile and Argentina meanwhile may choose to support a side in this dispute, but since Bolivia may be willing to back down earlier neither power is able to really influence the course of events. For a longer peace between the two countries the resulting treaty would again have to allow both nations to get equal navigation rights on the Paraguay River for a chance at economic growth. This scenario does depend on whether or not Bolivia feels it is ready to go to war and assumes that Paraguay feels it can go to war and takes the initiative, given its experiences in the Great War.
Yet another scenario is a full-blown war between the two countries, just like in our timeline, with a few factors being changed around. This one I feel is the worst case scenario, because it has potential to turn into a bloody proxy war between Argentina and Chile, given their sympathies to their respective factions.
I'd comment more in this last scenario, but it might be too long and may get a bit weird with who starts supporting who. Perhaps others may have their own take on it. I have my own.