Care to enlighten me on the other possible options? The way I was catered the cake was that they had either the Tevinter servitude or death to choose from.
Their best option is to stay put and do everything possible to help the town. They had an agreement with the Fereldan monarch for sanctuary in the single most defensible location in southern Theda's. As long as they don't rock the boat, they couldn't get a better deal. If the Templars were ever stupid enough to attack, the castle could hold until a heavily pissed off monarch brought a full army to bear on the Templars likely destroying the order. If the Templars don't attack, then they can meld into normal life gaining freedom.
If they make the assessment that the Fereldan forces could not route the Templars, then they have no hope at a military victory and they must scatter as they can never get better conditions for their rebellion. Saving as many lives as possible is the key then and not moving as a group is their best bet for that.
If they don't believe they can trust the monarch....then I don't get why they are in Redcliffe, so I am assuming that hypothetical is off the table (but if it was on the table, leaving is the answer).
What they did instead was created a new eny out of an ally (Fereldan) and replaced them with an ally who is not able to get supply lines or capable of holding the territory they took simply from a logistics perspective. Further, while we don't know the size of Alexius's forces, they have to be significantly smaller than the Fereldan army would be due to said logistics. In addition Alexius's forces are presumably mages....which puts them at a disadvantage as TI mages don't know how to handle their magic being blocked (war table mission confirms this). So she is trading swordsmen in heavy armor for mages with no magic.
So in summary, Fiona's alliance decreases the size of her allied forces, increases the number of her enemies, and her new allies are less suited for fighting Templars than her old allies. In other words, sitting on her hands was not only a better option, it was in fact her BEST option.