Matt84, in your argument you are considering Paraguay as a kind of successor of a 'Guaraní nation'. Guaraní heritage is present in Paraguay, Argentina, Bolivia and Brazil, with the Guaraní people having their share of trouble with authority in each of these nations. In Paraguay, their legacy is predominant, but I'm sure that Argentines who speak Guaraní don't consider themselves somewhat Paraguayan, to put an example. Unless they or their close ancestors are immigrants from that country, of course, but I guess welcoming immigrants shouldn't be a reason for losing territory...
The fact that the Guaraní language is official in Paraguay (BTW, it is also in Bolivia and the Argentine province of Corrientes) is not the reflection of a political representation of these indigenous peoples in Paraguayan governments. When South American nations formed, most of the Guaraní descendants were of mixed, Spanish descent. Many of these 'mestizos' spoke Guaraní, or rather a modern version that had been mixed with Spanish, because the Guaranís were particularly friendly to the Spaniards at the time of the 'conquest', thus the latter didn't crush their culture, and because the Jesuits worked on unifying different dialects, producing one that was more useful and, therefore, more resistant. National (or regional) pride supported the language through more recent times, sometimes spurred by government. There are people who speak Guaraní who have little indigenous ancestry, if any.
Just to be clear, a good portion of the territories that were gotten in the Triple Alliance war, mainly in Chaco and Formosa, were inhabited by other indigenous peoples. Some disliked Paraguay and helped the Argentine army.
I never heard anyone claiming that the British stole the islands from indigenous peoples, the claim is that they 'stole' them from Argentina. The 'extended continental shelf' argument is not the basis of the claim. The quid is that Argentina was the legal possessor of the islands when the British took them illegally in 1833. One of the reasons for not seeing remnants of Argentine culture is that they expelled the Argentine settlers and didn't allow more to go there. Some may argue that only the authorities and soldiers were expelled, not the settlers, but see
this response by Andres Cisneros.
For a number of reasons, the legal framework used to organize the world has disregarded indigenous peoples. Therefore, now that we are not looking down on these populations as our ancestors did, our sense of justice tells us that their territories were stolen, but attempting to revert that situation would be pretty much unfeasible. However, with Malvinas, the legal framework does apply and it is not impossible to work together on enforcing rights attributed by that framework. So, I agree that the islands were one exceptional piece of territory as they weren't 'stolen' from Amerindians. They were 'stolen' from a formal nation and that is why it corresponds to treat this case differently.
I believe there may be, hopefully, one parallel with the case of the northern provinces after all, because the region blossomed after the Triple Alliance war and the boundary treaties that followed. I'm not interested in chauvinist rhetoric, name giving or troubling the islanders, what I would like to have are conditions for a peaceful and straightforward settlement of sovereignty in the Southern Atlantic and Antarctic territory when its time comes, as it may come in a few decades -- the hypothetical Malvinas oil is most probably just the tip of the iceberg regarding to what the region has to offer resource-wise, given the increasing demand for commodities. We most certainly agree that all countries would benefit from a civil settlement, one that doesn't have us, or our descendants, wasting time, spending on the military and distracting attention from other problems. Just like South America benefited from agreeing on its boundaries 140 years ago. But for that we first need to talk about these islands and make historical and legal reasons better known.