The Deceive and Subterfuge skills may assist a character’s Inquiry efforts, if he first attempts to establish himself as a peer of the targets (p. 82)
I don't think it's meaning the rules for Assisting skills but rather a more general use of "assist." In other words, if you lie or disguise yourself as a friend before making an inquiry check, that will probably be worth a bonus, per GM discretion.
I think the +10 isn't a bad modifier, but as Nimsim says, it's going to be GM's discretion - depending on what you're inquiring about. I can't see how you can give a hard-and-fast-rule.
Saying "so....seen any tzeenchian heretics about?" is a dangerous question that needs to be posed very carefully, and using disguise or deception to make people think you're already part of the cult is going to help.
By comparison, I can't see how asking "Does anyone know a shop that sells decent body armour?" in the middle of a crowded market probably isn't going to derive any bonus from you waring an Inspector Closeau style false-nose-and-glasses.
Influence should definitely help Inquiry, since it represents your ability either to grease palms or to provide more abstract 'favours' that encourage people to be honest and helpful.