Q2: the passenger airbag doesn't come with deactivation function. However, if your warning light of passenger seatbelt doesn't on when you put your child seat and so on, then your passenger airbag should be no function. The seat have weight sensor and the child seat is quite light. I must read more on the manual book.
Q3: Mine also doesn't come with armrest. I remember the 2-doors 120 sport have it, but is permanent one, meaning it seats 4 only.