1: What is the "average" lifespan of: Inquisitors/normal citizens/imperial navy officers/planetary governors? i need an idea as to how old to make my characters in their background.
Average life expectancy:
Field Inquisitors would, on average, live to less than 60. It's a very dangerous job, and sometimes no amount of medical tech can save you from catching a lasbolt with your face.
Confusingly, the average age of a field Inquisitor is much higher than their average life expectancy. (The difference is because you're only counting the ones who are actually alive, as opposed to counting in all the ones who die young and drag down the other statistics.) Those who have got the blend of luck, skill and toughness that it takes can still be smiting heretics at three hundred or more.
Anywhere in the middle is possible. Desk Inquisitors are obviously expected to live longer, but they'll reach "old age" at about the same point (as most people do!).
Normal citizens - entirely depends on the planet and the class of citizenry. But as Octavian says, there are good real world equivalents. A feudal world would have a life expectancy like you'd expect in the middle ages and a civilised world would be similar to the modern first world. Most people don't have access to rejuve tech, so they live what would be considered a typical lifespan for a human under those conditions.
Officers: Varies. A high ranking one could be a good couple of hundred years old, kept alive for their expertise. Most - refer to officers in the modern military.
Planetary Governors: Could be a good few hundred - they've got access to rejuve, so provided they don't get assassinated, they'll be ageing at something like a fifth of the normal rate.
2: Is there any info on the colours of specific administratum robes, such as clerics/oficio munitorium/imperial navy uniforms/adeptus administratum scribes/workers?
Firstly, the Administratum is NOT to be confused with the Adeptus Terra.
The Adeptus Ministorum (a.k.a. Ecclesiarchy), Departmento Munitorum, Imperial Navy and even the lowly hive workers are not part of the Adminstratum.
Administratum robes (as worn by Administratum adepts/scribes) start as black and become increasingly white with higher ranks.
Ministorum: depends on sect, but think "Pilgrim" or "Monk" and you're not far off. White, cream, brown, grey. Maybe black, but probably not like you'd imagine a Christian pastor to wear.
Munitorum: wouldn't wear robes, I don't imagine. They're part of a military supply line, so probably olive green fatigues.
Imperial Navy: Navy blue, what else?
Hive workers: Anything and everything. There are tens of thousands of hive worlds, most of which have dozens of hives. Imagine asking "What colour do people on Earth wear", except a million times more vague. Probably not hi-vis jackets and hard hats though, the Imperium isn't big on health and safety.
3: any info or pics of adeptus arbites uniforms?
Look at what the riot officers are wearing in the news, make it a bit more sturdy, and you won't be too far off.
