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If there was any immediacy factor you'd already be dead/captured.
Logically, if at any point in the timeline Alexius could have stopped you using time magic, he would have jumped back and done so. The fact he has not, confirms that time magic is not a real threat.
In general this is why writers should stay away from time magic as a plot device, it's never done well, and this is no exception.
Of course, you are right - thanks for pointing that out, should have thought of it myself.
However, the fact that you can rule out Alexius ever achieving time travel to undo the Inquisition, does not rule out something just as threatening: Alexius achieving a way to stop time, locally. After all, what you've already witnessed is the slowing of time, and the improvement need only be incremental, to slow down time so much as to make you freeze in place.
If control over such magic was achieved by Alexius, all would be lost - he could just freeze the inquisition forces, and leave them that way, while his master gets his way unopposed.
And you don't know how far Alexius is away from achieving THAT. Could be years away, months away, weeks away, days away, or hours away. Thus not dealing with Alexius post-haste is playing with fire.
The immediacy again calls into question the wisdom of abandoning that path, and going after the templars, especially since the templars have shown no indication they COULD be convinced to help - their leader seemed quite unreasonable, and definitely hostile and dismissive of the Inquisition. For all the Inquisitor knows, the templar path might already be out of the question, and an effort to recruit them would only waste time. At the very least, the Inquisitor has to consider that a possible, even likely outcome.
The way I did it in the end was just to head-cannon it so that my Inquisitor was inexperienced with magic, and didn't trust her ability to deal with the situation, and going to the templars for help was a desperation move; one that backfired as she now discovered she cannot immediately go after Alexius with the templars after all. So she considers her actions to have been a mistake - and after seeing the templars fooled by a demon, she doesn't trust the templars enough to lead themselves, and subsumed the order into the Inquisition. But as she told Solas later, beggars can't be choosers, and she just has to make do with the situation as it is, and hope that Alexius isn't about to freeze all their asses.
It still felt a bit weak to me, but it'll do, I guess.