I don't think Fiona we saw in Val Royeaux was a demon - there wasn't any advantage for Alexius or Corypheus in luring inquisitor early, especially when everything was going according to their plan at that time.
I think that Fiona we encountered in Val Royeaux was a person from the unmodified timeline and from time before Alexius modified anything. By the time inquisitor made his way to Redcliffe Alexius arrived and started performing ritual, which caused a rift with time phenomena to open - and which incidentally converged with the time of arrival (we know the rift with time annomalies just opened because we see gate guardian running away).
Maybe because of the fact that a team was close to time anomally rift at the time of ritual (they were closing it) they somehow weren't affected with change to fit different timeline and preserved their memories (at least the group that was with Inquisitor at that time) of the "old" world. Since the "old" world and "new" world doesn't differ much for inquisitor we have memories from before time change, but there isn't any significant change in surrounding world to notice anything that wouldn't be consistent with it.
Or maybe we switched places with another inquisitor from different timeline. Who knows, maybe somewhere there is inquisitor who now wonders how it is possible that he arrived at the same time with Alexius at the gates of Redcliffe when he thinks that Tevinter arrived in Redcliffe few weeks earlier? And wonders why Fiona was expecting him. 
As for why bad guys didn't go back in time to kill inquisitor, or fix the ritual that started the game - we learn from notes in far future that Alexius can't go back to a time before first, biggest rift opened, which apparently is too late.
Also I think, that if we go to Templars instead of Mages we are affected with time change and our newest hero doesn't remember the encounter - only we, players, do.