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Sometimes petunias benefit from being cut back. They can get "leggy" with few leaves along the lower part of the stems and flowers only at the tip. It seems drastic but I would cut them back by 1/3. Make sure to have leaves remaining or the entire stem will die. Petunias don't require fertile soil and with as much water and fertilizer you are giving them, might be growing excessively. Follow the instructions on the fertilizer container. Water when the soil is dry: keeping the soil damp by frequent watering can cause root rot. Let the top 1 " of soil dry out between waterings. In hot, dry or windy weather daily may be necessary but check the soil first. Here is more information: https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-container-care.htm

What a chore it would be to dead-head these prolific flowers! Fortunately, it isn't necessary. As stems get leggy or flowering decreases, trim the stems back by 3-5 inches, fertilize lightly (half-dose) and a new flush of blooms will occur. If a stem has no leaves lower down, trim it all the way down.

I would need a photo to be able to see what is going on. I would say that overwatering is a possibility. These are drought tolerant, but can easily get a disease when overwatered. They can also be affected by getting water on the leaves and flowers in bright light, as well as infections from the leaves remaining wet.

In the meantime, this article will help: https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-care.htm

Unfortunately, a lot of petunias, surfinias included, don't smell very good, particularly when the flowers are fading. There's not much to be done about it, except moving them farther from highly trafficked areas or surrounding them with better smelling flowers.

This sounds like Mites. Neem Oil is a good treatment.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

https://www.gardeningknowhow.com/ornamental/flowers/petunia/problems-with-petunias.htm

Petunias like it on the dry side. Yours may be suffering from root rot due to too much moisture. I'm convinced that more plants die of too much water than too little but it is hard to judge watering for containers in very hot weather. Here is an article that discusses diseases that affect petunias.
https://www.gardeningknowhow.com/ornamental/flowers/petunia/yellow-leaves-on-petunia.htm