My first thought is that it wasn't an iron deficiency to begin with since you still have yellowing. My guess is that your watering is off somehow, either overwatering or inderwatering (yellowing) in conjunction with too much ferts (browning; salt damage). Once a week indoors strikes me as a bit much since you most of the time you are watering only once or twice a week to begin with.

My guesss.....
A total rebellion against enviromental shock and watering and food bad habits.

That poor thing.

Give it a stable enviroment. Outdoors in shade on your hot days is better than indoors.
To little water is better than too much.
To little fertilzer is better than too much..

In those extreme temps, they can't take up nutrients anyways, neither can they when temps at too night are to warm, nor can they when overwatering.

Therfore the yellowing leaves.

Can't you plant it in a soiless mix, if you havn't already, by the sound of it, you might not have, where you can water almost everyday, where it is virtually impossible to build up salts in the soil or get root rot when the roots breathe, that has great drainage.
I can bet you your denia will not rebel anymore..:-)
Just curious.....:-)