Black Orc Warboss on chariot question

Yes, that is the rule I was quoting indirectly.

And that's how I see it too.

I was just thinking that someone COULD argue that there are more models involved without the immune to the psychology rule... Orc crew, boars, the chariot itself... so that the unit as a whole shouldn't have it.

But I also see it that way that there ist just a rider (the Black Orc character) and a mount (chariot, crew, boars).

If I remember correctly, in case of Grom it is stated explicitly (in the rules of the FAQ?) that Niblet as the BSB is considered part of the chariot - so I'd say if a character is on a chariot, other crew members are considered part of the mount.

The part of the ItP special rule that states "If the majority of the models in a unit have the Immune to Psychology rule..." does not come into play, because "In a similar manner to cavalry, a chariot has more than one set of characteristics, one for the beasts, one for the crew and one for the chariot itself, and is treated as a single model" (BRB p. 86, Italics mine).  So, it does not matter how many crew members there are, it remains a single model. 

In addition, a character on a chariot follows the rules for "ridden monsters,"  in which the character is the rider and the chariot (crew included) the mount. As such, "special rules that apply to a ridden monster do not normally also apply to character riding it and vice versa (with the same exceptions that apply to cavalry models, which are listed on page 82)" (Italics mine).