1 - I play on Kong (where there is no DMU currently), my 50+ unique items say otherwise. Though for this plan, I can see why you need it.
2 - I have found fuel and unique items on the same mission, somewhere around 25% of the time, so no.
3 - Offices and Warehouses do not have any more fuel than any other location. More important is WHEN you start looting. The fuel cap is reset each day at the same time (there are several posts/threads in the forum which document this). With a good peak oil book (doesn't need to be 50%) you'll hit the fuel cap within the first few missions run regardless of location. How do I know? Well since I don't have the DMU I don't waste my time with offices, but I always hit the cap, at/near the same time each day. As far as making sure the books are working, yeah that's a given. Since I don't have the DMU, I do played missions when I use looter books. Knowing which containers my drop weapons/gear and which containers don't is very important. Go through all the weapons locations, then refresh, repeat.
4 - Apophenia again. I've gotten unique items on the first mission of the day, but I don't assume that if I'm going to get a unique that day, it's going to be the first mission of the day -- that's just illogical.
5 - Looting weapons locations a priority? check. Looting warehouses too? Well if you have the DMU sure, otherwise it's just a waste of time, warehouses have more weapons locations than all other non-weapon locations, but still less than ones where weapons are a priority.
6 - It has been said by Con that your chances of finding a unique go down if you find one. So yes don't waste temp books after you've found one. That being said, its possible to loot more than one unique per day (I've done it). It's even possible to loot two unique items in the same mission -- in the same container.
Looter books: Have about 30 in my inventory right now, none of which came from looting warehouses.
Shiny: ten 4% perms and bunch of temps (thrown out more temps than I care to count), none of which came from offices.
Hats: Played missions is 100% correct, you'll never get a hat on auto. And certain hats are more common in certain areas (UCPD hats in police stations for instance). Baseball caps and bandana's can be found just about anywhere though.