You are underfeeding.

Purple stems = cold temps and phosphorus deficiency
Crispy lower leaves = N-P-K deficiency.

keep feeding her 1/2 strength but every other watering, not twice in 7 weeks!

You should also really try and do something about those temperature swings. A 25F degree drop is huge, especially since growth virtually stops below 50F. I find a small (6") fan-forced heater is a godsend. I have a sunbeam model that keeps the room at a constant 76 degrees.

Good luck!