Hi again, Paolo --
First of all, I don't know what the "ball" is that you're describing between your stomach and breast, but a hiatal hernia can not be seen visually in this manner. The esophagus passes through the hiatus within the chest cavity -- not close enough to the chest wall to protrude. It's not like an intestinal hernia that can be visible through the skin...it's that the stomach can pass slightly above the diaphragm through the hiatal aperture. Completely different pathology.
As for your symptoms you describe:
"pain in the central part of my stomach" -- this could be any number of things, and could possibly be exacerbating reflux symptoms.
"feeling of burning in my esophagus" -- this could be heartburn, or other, related conditions.
"feeling of foreign body in my throat" -- sounds like you are describing globus sensation, a symptom of LPR.
Your other symptoms of throat and mouth issues are also reflux. I completely identify with how it impacts even your speaking voice, and I agree wholeheartedly with your singing teacher. DO NOT SING with these impairments!!! As you know, singing requires a special embouchure and strain, that singers train to control. The irritations from the reflux take away your ability to maintain the embouchure safely while projecting your voice, and further damage can occur as you try to compensate.
Before I understood what was happening to me, my voice started slowly collapsing, over a period of months. My upper register took more effort and strain, and those notes started disappearing. I could no longer support softer notes for any length of time, because my vocal cords wouldn't close and limit the amount of pressurized breath escaping from my lungs. I could no longer complete a repertoire of songs before my voice would completely collapse, until I couldn't even finish one song at a stretch. I began popping throat lozenges. Trying to sing also caused a stinging irritation in the back of my throat as well, below the palate, presumably because this is where most of the acid would burn as I sleep on my back.
On a good day now, I could probably sing "OK" for about 30 seconds or so, but I will be hoarse for a couple of days afterwards. I can't even speak for long periods of time before my voice begins getting "gravelly", something that ENTs call dysphonia.
Any of this sound familiar?
The good news for you is that chances are good that you can find relief without surgery. You need to make changes in your diet, and how you eat. There are certain trigger foods that may make your reflux worse. Do not eat or drink within 5-6 hours of bed, and raise the head of your bed in the neighborhood of 10-14". I find this easiest with a special foam wedge that starts at my hips. Don't just use pillows -- as that will make it worse. I'm 6'1", and I need to augment the wedge with folded towels and blankets to support the pillow at the top.
You have a formulation of Gaviscon available in the UK called Gaviscon Advance, I believe. Take this product before bed. It won't protect throughout the night, but it will limit your exposure. Also, while you're at the drugstore, talk with the pharmacist and start with what is known as an "H2 blocker." There are several to choose from, and these should limit the acid your stomach produces. There is another line of defense available to you if the H2 blocker doesn't work, called a PPI, or Proton Pump Inhibitor. There are over-the-counter PPIs available, and other, stronger formulations available from a doctor.
What you're doing here is some preventative medicine, as well as some investigation that a doctor should find helpful. If you improve using what I've described, it will help a GI doctor quickly zero in on a course of treatment. You need to find a doctor you can trust, to help you sort through all of this. This "ball" you described, as well as the stomach pains, need further investigation. Surgery is a last resort -- don't rush it!
Hope something here is helpful.