My thoughts exactlyNo, you may be right. A connection between these various eye symbols is very possible. I mean, the interelopers are portrayed as shadows of a Hylian and their magic as an eye symbol while the Sheikah are described as shadows of Hylians and also have an eye symbol.

I've considered that... the Sheikah could have managed Arbiter's Grounds as guards... but there's a little problem: the eye symbol on the shield doesn't have a tear. Generally when we see the eye symbol used by the sheikah it has the tear whereas, when it is used by others like Agahnim, Veran or Ganondorf, the tear is not there.There definetly could be a connection. I mean, considering the Twili banishment seemingly happened before OoT, the Stallord's soldiers could be the Sheikah who became guards of the Arbiter's Grounds. While that's never said, it could be a possibility.
And since, as you just mentioned, the Interlopers used dark magic, represented on the fused shadow with a similar tearless eye, and they appeared among those living in the light (which would suppose they did not, live in the light), I wonder if it couldn't be that despite what it would seem like from cutscene dialogues (and lack thereof) the Interlopers were indeed kept within Arbiter's Ground previous to being banished and that those soldiers protecting StalLord were interlopers who died in there.
I don't believe this to be the case, but I think it's a thought worth pondering.
I didn't mean to say that the symbol in Forsaken Fortress was that of the Sheikah, but that it represented Shadow (Dark?) magic (similarly to how Koume and Kotake use the symbols on the fire and water medallions for their fire and ice spells).But some things, like the symbols on the Forsaken Fortress probably have nothing to do with the Sheikah because I can't see how it would.
Welcome to my world...For fuck's sake, defensive much? I just asked what the connection between Shiekah and Stalfos was. IT WAS A GENUINE QUERY.

But anyway, Stalfos supposedly were something else before being Stalfos (unless we assume Hyrule has unnatural born skeletons)