You're over-thinking it the way Mass Effect 1 players do who defend diverting resources to save the council. I go by what is presented in the war room in regards to acquiring allies to stop the breach with the least danger. Cullen makes it clear. "It could work but it's a huge risk" referring to the mage plan. The templar plan's discussion wields almost no risk at all.
Of course they don't find it risky, because none of the advisors have any idea what's going on at all. You walk into Therinfal completely blind. At least we can feel lucky that the archer caps Abernache instead of the Inquisitor in the cutscene.
Of course, we'd (or, the advisors) also be assuming that the situation in Redcliffe will just remain static this whole time, which is dumb (and the factor that made me choose mages first playthrough). By this time we know for a fact that the Venatori are mass murderers, who have some control over the rifts, plan to take control of the rebel mages, are some kind of Tevinter-supremacist sect, and straight-up intend to kill you. For all we know, they can sack Redcliffe or move straight toward Haven while we're off traveling to Therinfal. Or both.