I'm glad you spotted what Jim said about the Merlin, I totally forgot he said that.
For a long time I have thought that if the Merlin wanted Harry to attack the Red Court, it wasn't because he thought Harry would destroy them, it was so Harry would create enough damage before going down, the White Council would be able to mop up a weakened and disorganized Red Court. Essentially Langtry set up Harry and company as suicide shock troops, thus (attempting) to kill two birds with one stone.