I agree a higher quality diet is important. NLS Thera A is my staple food.
The heat treatment works without the use of salt, so IMO there's no need to add to the stress of the fish by adding it.
Ich will spread to the rest of the body, while other white spots do not. The spots look a little large to be ich, but not so much so that it rules ich out. Not necessarily the advice you were looking for probably, but I would wait and see how it develops. Ich is an ailment for which you have lots to time to treat, so you have the luxury of waiting and seeing.