Although I have very bad digestions when I tried betaine or digestive bitters i get some heartburn. I think my digestive problems come from candida, their toxins, lack of lacto/bifido bacteria, etc... because I also tried digestive enzymes with no results.

Candida can contribute to heartburn, but this still goes back to a lack of stomach acid. All of the areas of the body where Candida normally resides are acidic because the acids help to control the Candida. Stomach acid is no different. When the stomach is sufficiently acidic Candida cannot grow in the stomach. When stomach acid levels decline or are neutralized with antacids, acid blockers, alkaline waters, calcium or magnesium hydroxide, baking soda, etc. Candida can grow in the stomach due to the alkalinity that promotes Candida growth and aggressiveness.  The Candida increases fermentation in the stomach leading to gas formation. As the gas escapes rapidly up the esophagus traces of acid are carried with it leading to the heartburn.

 Although yeast causes gut deterioration (like English Ivy destroying a brick wall),

Candida DOES NOT cause damage to the intestinal wall in its yeast form. The damage from Candida comes from Candida's fungal form, in which it develops hyphae, finger-like projections that allow the Candida to dig in to the tissues and cause damage. Alkalinity promotes the conversion of Candida in to its fungal form and turns on the Candida growth gene. Acidity turns off the Candida growth gene and keeps the Candida in its benign yeast form.

Many of the yeast byproducts are acids and release of the acids that are absorbed into the body may cause a condition called metabolic acidosis.

This is very misleading. Acidosis is a very rare condition since the body has numerous methods of maintaining its pH preventing it from becoming too acid or too alkaline. This is because the body can only live in a very narrow pH range. Therefore any acids generated by pathogens would be quickly neutralized by the body.

In addition, the body is highly dependent on a number of acids including hydrochloric, lactic, acetic, fatty, glucoronic, hyaluronic, uric, bile and carbonic acids to name just  a few.

The most convenient product is a nonprescription drug called AlkaSeltzer Gold™. Do not use any other kind of AlkaSeltzer™. AlkaSeltzer Gold™ is simply a very safe product (sodium and potassium bicarbonate) that helps to neutralize excess acids of any kind. 

It is not safe to either neutralize stomach acid or to try and force the body in to an hyperalkaline state.

The restoring of acid/alkaline balance also relieves many allergies.

Again this is not true. The body maintains its pH balance very well except in rare cases such as poisonings and when people try to force their pH in to the alkaline side. And allergies have NOTHING to do with acidity or alkalinity. Allergies stem from adrenal dysfunction. When the adrenals are not functioning properly there is a lack of antihistamine and antileukotriene epinephrine and immune modulating and anti-inflammatory corticosteroids. This is why we are all exposed to the same allergens, but we do not all have allergies. Healthy adrenals prevent allergies. The same reason that babies born with allergies can outgrow their allergies around the age of 5. The adrenals need time to mature.

And this is what I suspected because when I took a combo of calcium/magnesium (in a mix of: carbonate/oxide,citrate & gluconate) I get some relief for this "acid" symptons (wich also causes some kind of anxiety..) probably because is poorly absorbed, or also because it does some effect after being absorbed.

Calcium and magnesium and especially oxides are not good for the body. Both will neutralize the stomach acid, which will lead to all sorts of problems including nutritional deficiencies, allergies from undigested proteins, Candida overgrowth, infection through the gut from other pathogens, acid reflux, etc. The oxides are also poorly absorbed and very caustic. This is why magnesium oxide in particular is used as a laxative. The magnesium oxide reacts with water to form magnesium hydroxide. This burns the intestinal wall leading to an influx of water in to the intestine and increased peristalsis. Although continually burning the intestinal wall also significantly increases the risk of intestinal cancer.

But it is weird, because if I took more then I get more die-off... so, Im pretty sure that you can kill candida with alcalinizers (calcium/magnesium/sodium/potassium... bicarbonates) but this seems to be a bad idea because of this acids. Killing candida with another substance and neutralizing the acids with "the only necessary amount" of bicarbonates may be the key. Otherwise you will enter in an endless loop.

The problem with this is that Candida growth and aggressiveness is increased by alkalinity.

 Too much, too little - which? Copper in relation to...zinc (or...)? Do you know?

Both copper and zinc inhibit Candida and low levels of either can contribute to Candida.

 Mmm it sounds me more like "irritation", in fact i feel a bitter/acid sensation but maybe also there is a little inflamation... but what is the cause? can be from die off?

Inflammation is usually from infection and/or trauma. Infections can cause trauma, such as Candida when it goes in to a fungal form from alkalinity. This is more likely since we all have Candida and it is constantly being killed off by the immune system yet we don't get these side effects.