My thoughts are similar. I like the part about Lum and Ry moving close along one wall, and then Ninja-boy can move up the other side and once the lightning strikes, the two sword swingers can do what they do best.
Now as to Mairead sneaking up behind them, I don't see her doing that. To go to the door and possibly there be more of them things come out or to get separated from the group? Besides I think we need to kill these things. We can't run from everything, I don't like the idea of leaving these things behind us and cutting off our only known way out.
No, she would rather stay mobile where she can aid whichever party member she needs to with her heals. She can heal by touch and has a healing aura which works on all allies within 10 yards.
Question for DM: without our flashlights, would there be sufficient illumination from the light and the firebugs, to move down the stairs slowly and make our way closer to the bugs without them spotting us?