Here are some possible answers to the problems you are having. Some plants are sensitive to insect sprays, so I usually test on one leaf before using on the whole plant. I don't know if this applies to hydrangeas.

Potting soil that plants come in often isn't the best long term medium, and those pots look way too small for the size of plant in a normal, non-nursery situation. Your type of hydrangea needs even moisture, so I'd repot in a larger container with a potting mix that is coarse enough to drain well, but with enough organic matter to remain evenly moist. The Container Gardening Forum may be able to help with this. For right now until you've repotted, you don't want the soil soggy or dry.

I would give your plants, which look like a cultivar of Hydrangea macrophylla/big leaf hydrangea, sun in the morning before you go to work, and perhaps after supper or in the very late afternoon, but leave them indoors during the day since they aren't plants that like full sun, but also won't bloom well if kept indoors all the time.

Ulitmately, these will be extremely high maintenance plants in your setting. I really try to buy plants that will do well in their setting without extraordinary effort from me. So for a sunny balcony, I would use sun-loving plants such as mandevilla vine, Salvias, Pelargoniums/geraniums, etc. or get a sun-loving Hydrangea such as H. paniculata Bobo.

It looks like a shrub whose root ball is starting to dry out and needs to be dunked in water for several hours to re-hydrate. I have one suffering of heat and water issues similarly. The drip got clogged and water was not getting to it.

The water deprived plants begin to shut down the blooms first, by making them turn brown instead of going thru the usual color changes.Then the leaves wilt and get sad looking like in your 1st pic, ending in stems that brown out too.

It typically flowers like it normally does in the States. Meaning it blooms only once in the Spring and the blooms last/stay on thru winter. You can deadhead the blooms when they have gone thru the color changes and end in brown blooms. However, there are mophead versions that rebloom and I do not know if this is one of them.

The south of Spain can be stressful for hydrangeas weather and soil-wise. Pontevedra and Galicia have better hydrangea weather.

A bigger pot might help too.