There was a thread about who it could be, which is worth checking.That thread is here.
There appears to be a lot of confusion about the plot, especially the events behind-the-scenes, so here is how I see it:
In the mid-19th century, an explorer called Walker leads an expedition in the jungle of San Theodoros and meets the Arumbayas who give him a fetish as a token of friendship. Lopez, an interpreter, steals a diamond from the Arumbayas. He hides the diamond in the fetish: the head can be loosened and the body is hollow enough to make a hiding place.
When the Arumbayas discover the theft of the diamond they attack the expedition. Lopez and Walker are the only survivors and manage to escape. Walker, unaware of what Lopez did, returns to Europe with the fetish which is eventually donated to the museum.
Back in San Theodoros, Lopez, who suffered a serious injury, is dying and writes a confession stating that the "diamond is in the fetish with the broken ear".
Several decades later, in San Theodoros, Rodrigo Tortilla finds the confession. Some research into Lopez's life enables him to identify the fetish in question and he goes to Europe in order to get it and the diamond. During the journey by ship he loses the piece of paper and it is found by Alonzo Perez and Ramon Bada. They do not understand what it means but the word "diamond" is enough to intrigue them.
Tortilla, Perez and Bada do become acquainted during the journey to Europe but the latter two do not connect Tortilla to the piece of paper they found.
Tortilla steals the fetish from the museum. He then gets Balthazar to make a fake. It is unlikely that Tortilla tells Balthazar about the diamond, but Balthazar is intrigued as to why Tortilla has gone to so much trouble. Balthazar may have suspected that the fetish was of some value and decided to keep it in the hope of getting something out of it: he lived in a loft and was not regular with the rent according to his landlady so he did have motives for this. He therefore makes two fakes: one to be placed in the museum and another to deceive Tortilla. The real fetish is hidden by Balthazar in a trunk.
Tortilla then murders Balthazar. He puts one of the fake fetishes in the museum in order to get the police to drop the case - which they do. Tortilla then leaves for South America, unaware that the fetish he is taking with him is also a fake.
Alonzo Perez and Ramon Bada learn about the theft of the fetish and, when it re-appears at the museum, notice, like Tintin, that it does not have the "broken ear", which is mentioned on the piece of paper. They also learn of Balthazar's death and conclude, like Tintin, that he made the fake. Through the parrot they learn that Rodrigo Tortilla murdered Balthazar.
Tortilla is making his way back to San Theodoros with the fake fetish but is murdered by Perez and Bada on the "Ville de Lyon". While they and Tintin battle it out all over South America, Balthazar's brother finds the real fetish in his late brother's trunk and uses it as a model for his wood carving business. He then sells the original to an American collector. Perez and Bada try to steal it from the collector but are caught by Tintin; the fetish is broken and the diamond lost to the sea.
Does that clear things up?