Define mortals.
Chaos followers are mortals, and may well be human, in which case they're not subject to the Emperor.
Eldar are mortals - when they die, they go into the Soul Matrix, where their knowledge goes towards aiding those capable of accessing it.
Obviously, most of the Imperium are mortals (being humans).
Tau are mortals?
However, I'm going to assume here, that you mean of those loyal to the Imperium.
In that case, it's one of three people, the Lord of the Departmento Munitorum, which commands one of the five sectors of Imperial Space, and through the hierarchy in the military chain of command, commands every single front line soldier, one day recruit or military based administration.
For the civilian side of it, which could well be the larger section of the Imperium, (common sense states, anyway), the Lord of the Administratum owns the majority, as he does the exact same as his Munitorum counterpart, but for the civilian parts.
Arguably, the one in overall command of them is the Lord Custodiann, the Emperors representative on the council of the Twelve High Lords, rumoured to be Constantine Valdor - if a case requires the vote, then majority rules - if theres an equal tie, and one of the sides includes the Lord Custodian will have the decider, if he so wishes. Alternatively, he can order a revote.
As to the salute, I have no idea, can't say I've heard of that. There's "making the sign of the aquila", which I assume is equal in size to the crossing of oneself in todays world.
Never read the book, so can't answer the next question.