That sounds like a pretty solid theory, makes a lot of sense!
Maybe the S elements refer to secondary elements somehow connected to the primary ones, Surge would likely be lightning based on the name, Strike might be an offshoot of Force's connection to Earth, Sunder might be closer to Foundation, maybe like a quake ability? Maybe metal, or perhaps poison or otherwise corruption, (I'm still shooting for there being some sort of undead element mixed up in this) and Subdue would most likely be closely connected to Fount's water element, forming an ice element... Oh I think I get it now.
The primary F elements are our primary four classical elements as you said,
And the S elements might be formed by a combination of the 4 main elements, creating a group of secondary elements. Maybe the F stands for First and the S stands for Second, separating the elements into primary and secondary groups, as Skiwiks showed in his diagram. And if we combine the edges of the blue diamond, they create their closest corner of the green square, so in other words:
Fury (Fire) + Force (Air) = Strike (Not entirely sure on this one, gonna need some suggestions!)
Force (Air) + Fount (Water) = Surge (Lightning) Makes sense because I think air plus water moisture does play a roll in creating lightning. I initially thought it would be Fire and Air but this makes more sense now that I think about it.
Fount (Water) + Foundation (Earth) = Subdue (Ice) Basically water taking on the characteristics of solid earth makes Ice
Foundation (Earth) plus Fury (Fire) = Sunder (Poison? Lava? Melty stuff? Not sure.)
And going one step further, maybe Muddle is created by combining any opposing elements in the primary group.
Force (Air) + Foundation (Earth) = Muddle (Erosion?)
Fury (Fire) + Fount (Water) = Muddle (Steam?)
Radiant (Light) + Murk (Dark) = Muddle? Maybe they just glance off each other though.
Maybe Muddle is just representative of a dud, Air and Earth might just break down the rocks over a very long period of time, and Fire and Water just make steam when combined with each other. Muddle must represent any sort of cancellation or lack of effect of the elements.
I don't think this would effect the secondary elements as well, but maybe we could still view those secondary elements as opposites of each other? What's the opposite of lightning? Maybe we could find out what Sunder is based off of that, and what's the opposite of ice? Maybe Strike represents some higher form of fire that we haven't thought of yet... Probably not lava or magma though, I don't think Fire and Air produce anything except a more well-fed fire (though magma is considered lava when it reaches the surface and is exposed to open air... I really don't know where I'm going here).
Alright, that's all I've got for now!