>Am I right to assume that Scions too are sensitive to these
>fluctuations of magic, similarly to Nexus?

Currently, no, except those that can already tell as part of their class abilities or otherwise. (Nexus gets to be a little special due to the story to this point and ultimately the nature of their cabal area; the swings of magic also affect them a little more strongly than most characters.)

My ultimate plan is for them to gain a little more help in this respect from their cabal area, and for some forms of their divination power to provide additional information. These will probably be loose ends dealt with by the Scions in one way or another soon.

The disposition of the Ebony Scepter, of course, is material to the fluctuations of magic, as are the state of the omens the Scions read...