Let's only bitch at Homura about her warning in jest, please. We don't know what kind of intel she was operating off of. Possibly only 2ndhand, which makes it 3rdhand when told to us.
Here's a thought, assuming grief->light is possible:
We think the enemy is going to continue operating at the edges of our range. If we set up grief marbles out there near the circumference of our control range, then detonate them into light bombs whenever we detect anyone enter. If we catch them and blind them or disrupt their attack, great. If we don't, we at least illuminate them, which is a nice consolation prize.
The only real disadvantage is a halo of marbles might delineate our control range to our enemies, but we're already on a huge honking grief fortress in the middle of it, so we aren't exactly being subtle about our range as is.