Why is it a mystery ?
Whenever there is a high risk there is a mandatory escort. Very simple and nothing to do with bribes.

Tunisia have terrorist groups near the border of Algeria (Chaambi mountain).
The same applies with the western part of Algeria where there is an ongoing war between the Polisario front and morocco.

South and East its even worse with Libya, Mali and Niger.

Basically anything above Ghardaia and far from the borders is fine.

Regarding Abdou Semmar, I was reading his articles since Algerie-focus website.
His "investigations" do not bring anything new to the table.
He is probably being paid by moroccan secret services ( I have nothing to back my claim with but from what he says, it becomes very clear)

Chengriha is not someone who likes power, he definitely hates morocco, he was in charge of the third military region (Bechar and Tindouf provinces), he has a lot of experience and wouldn't hesitate start a war with Morocco in order to destroy it.

Tebboune has a lot of power because he is the president and can remove Chengriha anytime.
Moreover, Chengriha is not that powerful within the army itself, he is starting to grow in power but he is nowhere near Gaid Salah in his prime for instance.
And even Gaid Salah in his time was not that powerful, Hamel, the head of Police was a serious threat for Bouteflika and Gaid Salah but he was put in jail after the cocaine affair in oran port.
This affair itself shows you how power was split, basically, you had police forces who were fighting against gendarmerie to get first to the port in order to seize the 701 kg of Cocaine.
Gendarmerie came first, Hamel was finished.

Why because in order to have the military with you, you need the head of intelligence, marine, DCSA, Police (which grew a lot in power) ... etc.

Boutflika was smart enough to know how to negociate with some military figures in order to eliminate the other ones and that is how he stayed in power.

He first got rid of Lamari by making an alliance with Toufik who was the head of intelligence while Lamari, Nezzar and Bellkhir were backing Benflis.

Algeria is really complicated and doesn't have a single point of failure like so many other countries (Gaddafi/Libya, Saddam/Irak... etc), if someone goes that doesn't change the rules, people change, the system stays the same, but no one knows who is in charge because power is not in the hands of a few people.