The Cassinite Heresy has now been disproved by analyzing the star patterns.
So, I've been exploring the possibility of the OTC taking place in Cassinite Madagascar in more detail. In order to not confuse people, unannotated cardinal directions refer to real-world Madagascar; Cassinite directions are marked with a subscript c.
Based on what we know so far, the most likely scenario given that assumption is that Cuegan's journey began on the beaches south (eastc) of Morondava, Madagascar. They traveled south (eastc) down the beach. There is very little in the way of human impact to encounter along the way, just a handful of small, isolated villages that, depending on their exact route, they may have missed entirely. The most likely river to be the OTC river is the Mangoky, which does have a large terminal delta similar to what we saw in the OTC. The Mangoky is also a fairly fast-moving river, especially upstream from the delta, and not at all the sort of thing one would wisely go swimming in.
Turning left, they then walked more or less alongside the Mangoky, going southeast (northwestc), then east (northc) along the river. Especially for a river its size, there is surprisingly little civilization on the banks of the Mangoky. The suspiciously flat patch of ground they walked over at one point may have been one of the few exceptions: a road, Madagascar Route 9, just south (eastc) of Ankiliabo. The bit where they had to go around difficult terrain is a relatively steep-walled valley where the Mangoky cuts through an isolated section of mountainous terrain between its confluences with the Sikily and Sakanavaka Rivers (both of which meet the Mangoky on the opposite side from where Cuegan are walking). That would make place them currently in the hilly terrain west (southc) of the commune of Beroroha. The real mountains are on the other side of the commune, the broken escarpments of the Madagascar Central Uplift. I suppose it should be noted that, if this theory is correct, that squirrelpy was really a lemurpy...
I think it's reasonable to assume Cuegan aren't natives. Perhaps they were tourists? The beach at Morondava gets great tourism reviews! This might explain their lack of area knowledge. What bits of information they seem to have gleaned about their surroundings would fit this scenario, too. The "hill people" upstream from Morondava would be ethnic Sakalava, while the area they would be in now is populated by the Bara people. Even their strange reactions to sea level rise might make sense if this world was originally Earth, and was subjected to the Cassini rotation in the relatively recent past; even if they have some knowledge of reality, no one sane would guess that sea level rise was caused by the catastrophic melting of neotropical Antarctica!
This theory, likely to be branded the "Cassinite Heresy", I am certain, does leave a few questions unanswered. For one, where are all the people? Even if they've avoided civilization since leaving, Morondava's not a small place (population ~70,000). Perhaps this is a consequence of whatever caused the Cassini rotation? In any case, the one other person we've seen seems to show French cultural influences, which would be appropriate for Madagascar as well. Another problem is the situation for any of the rivers except the one they have walked along. In our world, the Morondava River has no problem reaching the sea, and neither do the Maharivo, Lampaolo, or Maintapaka Rivers that they would have encountered before reaching the Mangoky. I posit this as the Riverine Paradox, but do not think it is an insurmountable challenge to the proposal, which otherwise satisfies both botanical and geographical evidence and provides a catastrophic global change in the recent past to explain some of the post-apocalyptical aspects of the OTC.