Regarding the explanation of Sotha Sil's appearance; Sotha Sil's silence was meant to have a much more poetic meaning. It is implied by his silence that he wanted to die, and as such he didn't put up a fight when Almalexia attacked. If Sotha Sil were to have protected himself, it is probable that Almalexia would have been killed, or at least had immense difficulty in killing Sotha.
To say that the silence was on account of it actually being a decoy would entirely destroy the poetic implications behind his murder. Not only that, but it seems unlikely that the wise and powerful Sotha Sil would avoid an attack, much less leave a decoy, and much more less when confronting a fellow Triune.
This kind of situation has sprung up a few times in the making of my mod - trying to create believable situations whilst not destroying the , as you put it nicely, poetry of the arc. This is the interpretation I took when I first found out about it and Seht's behaviour in cutting himself off from the world could back the situation up. I understand what you're saying.
I think it is almost necessary for random Dwemer ghosts to be added that are non hostile, that you can talk too for information, or charm.
I have to be careful about how much of the Dwemer's history I reveal/surmise. In creating the mod I've had to be careful about not destroying the enigma of the Dwarves as that is their charm. Originally, I had symmetrical rooms and each room's purpose what made clear to the player but I found that they became just another race to me and lost a lot of their mystery - it had been explained away. I also chose not to allow the player to learn to translate their books as this would also take much away from the feel of the Dwemer.
As it stands I have tried to preserve that feeling you get when you enter a Dwemer ruin - that its an enigma wrapped up with fear, excitement, an age gone by of the unknown. To have too much information about their past would be to break what makes them so fantastic.
That said, a one or two like Radac might be a nice touch.