Rust spot on leaves

This is due to the low ph. the leaves appear at the beginning of a drop in color, yellow spots that eventually turn into a burn such as in your photo. such an effect also gives a sharp drop in PH from high to low or vice versa. in your case, it's the low ph that caused the feed blockage. if you have already washed the plant, then it is important to dry the soil to 40-30% and then give basic fertilizers + kalcium and magnesium and be sure to set the ph level to 5.8-6.2 damaged leaves will not heal but new symptoms will not appear ..

Guessing to much food and its just taking a while to show results. or your light is to close. Try raising it up by 5cm to 50cm and see how they react. I don't think it looks like calcium at all.

Yes it is possible. Your plant is in the early flowering stage, so it also needs more phosphorus. I would also check if the lamp distance is right - not too close.