The game being on PS4 means far more to EG than it does to Sony, thus giving Sony more leverage to push the deal through on their terms.
You can say EG are complicit and should share some of the blame but nobody can expect them to sacrifice a huge revenue stream for a principal.
If you do want to apportion blame to both then it should be something like 99% Sony - 1% EG.
If EG were now to approach Sony & give them an ultimatum of including untethered accounts or lose Fortnite, I wouldn't be surprised if Sony told them where to stick it, just to show other developers/publishers that you don't fuck with Sony on their platform - will keep others from trying something similiar in the future.
And to your point about the situation being complex - it's not really.
Sony has weighed up the pro's (an attempt to reduce the number of players spending time & money on other consoles) and the cons (pissing of everyone who owns a PS4 & Switch, plus anyone who made the mistake of logging in to PSN without any intention of making the PS4 their preferred platform of choice) & decided that the former outweighed the latter. Simple.
And now they're reaping the criticism that they duly deserve.
Well if it is as you say then that would be very bad news for those pissed off by this controversy.
You and others are basically saying that Sony is not afraid of what could be the biggest backlash in gaming history by completely removing Fortnite from the Playstation if Epic Game decided to not play ball and agree to block Fortnite accounts on other systems. We are talking about telling some 30 million Playstation players (Probably more) of Fortnite that their favorite game is gone from their Playstation.
So if such a huge shit storm does not scare Sony then imagine how little they will care about the current fortnite account being blocked on Switch backlash. In other words don't bother appealing to Sony cause they won't care cause they're totally not afraid... Or at least that's what your theory entails.
I'm sleepy, time for a nappy!