First of all, bitter olive oil is not acidic or spoiled. Oil acids are not bitter but sour instead. Therefore, oils don’t get bitter while they go bad. They go sour instead.
Bitterness comes from polyphenols which show that olive oil is good. If an olive oil stings your throat and tongue it shows that it is earl harvest. This means the olive oil is preserved well on contrary to being spoiled. Good olive oil leaves your mouth clean and fresh. If it makes your mouth feel oily, that means olive oil is old and was left open.
No matter if the free fatty acidity of the olive oil is 0,3% or 33% its PH value of approx. 5,5 stays the same. This PH value is suitable for human consumption.
It is a misconception that olive oil with high acidity has a stinging sensation because olive oil acids are water-insoluble.
Instead of saying acidic or not, it is better to say “free acidity measurements are so and so”.
When it comes to the bitter oils, yes there is a concept of oils going bitter. Oxidation makes oils go bitter. Nevertheless, olive oil is different from the other oils because it already has a bitter taste coming from the olive fruit itself. This bitterness in olive oil shows that it is early harvest, processed well and stored well. Lastly, stinging oils. Since stinging sensation only happens in early harvest olive oils we can say that oils with this sensation are good