So the templar army was two days away from Redcliffe...and what's your point exactly? Neither the inquisition nor the ferelden army noticed how this massive templar force marched against Haven. But Fiona is incompetent because she didn't do some scouting mission?
No Fiona's incompetent because she didn't do ANYTHING! The Vints showed up out of thin air (literally) and said "Hey a massive templar force is just outside your walls and they're coming to kill you, don't ask how we know, just trust us. All our help costs is ten years of slavery and your children fighting our war with the qunari, it's a steal." And Fiona went "Ummm, that's not good. Ok, you have yourselves a deal."
All she had to do was NOT take the Venatori at their word. Ask for proof, how they knew, as long as she did SOMEthing!