The real question, with regard to the River Kingdoms, is how so many major rivers start there when there's no notable mountain ranges, no notable lakes, and it's too far inland to get a lot of rainfall.
Thats an easy question to answer. The Sellen River is fed by the Lake of Mists and Veils. The Lake of Mists and Veils is a glacially fed lake, in the manner of the Great Lakes on the US-Canadian border.
I propose that there was some glacial expansion during the Age of Darkness (about 1000 years long), in effect making it a minor ice age. It would be way shorter than a glacial period (which operates on the scale of tens of thousands of years) but longer than the "Little Ice Age" (roughly 500 years). The ice age could have been accelerated by the magic behind Earthfall. The warming that has been occurring since has been melting away the glacial sheet on the north side of the lake and feeding the Sellen River.
I don't mean the Sellen, East Sellen, West Sellen, or Shrike rivers, I mean all the little ones. It seems implied that there must be a small range of mountains in between Daggermark and Sevenarches, and possibly extending all the way north into Numeria, because otherwise the river which passes through Daggermark would flow east to join up with the Sellen, and the tributary south of Sevenarches wouldn't flow fifty miles east into the Sellen when there's another river less than ten miles west.
While on this topic, I would like to point out how utterly confusing it is that ALL the rivers are called Sellen. Apparently, even the river which flows through Daggermark is called the Sellen River, as the Guide to the River Kingdoms describes Daggermark as sitting on a hill overlooking the Sellen River. Both the West Sellen River (which originates up between Mendev and old Sarkoria) and the Sellen River (which originates in the Lake of Mists and Veils) are 60ish miles from Daggermark. From the top of the Burj Khalifa in Dubai, the horizon is 64 miles distant. There's no way you can see either the West Sellen or Sellen from Daggermark unless there's a fourth Sellen River.
Ah, I was confused when you said major rivers :) Sorry.
There are probably some hills in that area. Or it could just be a general rise in topography - for example a low ridge that isn't really noticeable as you go up it (but is enough for water to flow down). The Goldenrun River seems the most likely candidate for hills around it as its headwaters are <25 mi from the Dagger River.
Otoh, some forks of the Mississippi come <25 miles from some of the headwaters of Lake Michigan in Illinois and Wisconsin. No mountians there and they seem to do fine :)
I'd look at a topographical map of North Dakota. There are three major river systems in that state - the Mississippi, the Missouri and the Red River of the North. All have headwaters in the state and there are no mountains in North Dakota.
As for the confusion im sure the locals can tell them apart. Seriously lol. Come to WV. We have a "North Fork of the South Branch of the Potomac River" here. I kid you not. We also have a "Middle Fork River". In reality this is the middle fork of the Tygart River... but the left and right forks aren't named left and right... they are named other things.
So I would agree completely with you about the naming conventions. Its a great gripe to flavor your character with!