1) behind someone's back 背後で(あまり良い意味では使わない) If you do something behind someone's back, you're doing it secretly and it's always something negative. Like if you plan a surprise party that's offered secretly, but you wouldn't say you've done it behind someone's back. Because it's not usually a negative kind of a thing. There are some other phrases related to "back" and most of them relate to bad things, so for example "to stab someone in the back"
able to protect themselves. It's a sneaky kind of an attack. If someone is on someone's back, it means that they are criticizing them harshly they won't leave them alone.
back stabber 裏切り者、わざと悪口を言う人
2) "Dates back to" is a phrase that means looks back to a certain point in time. You could say "dates from" instead. "Dates back to" is strongly routed in now looking back over the past. Right dates back to some point in the past, "dates from" is much more they're feeling of coming from the past up to now.
3) Takahashi-san could have said what's the annual employee turnover. It has a same meaning as in yearly numbers.
successful selling lots of cars, because they are offering the employee discount to anybody who came to buy one,
5) Laurels 月桂冠(→名誉、栄冠) Laurels are leaves formed into a kind of wreath and used as a symbol of victory or maybe of doing a good job. So sometimes
So if someone is sitting on their laurels, it means they're no longer being active in building or maintaining their reputation or their victory. They're just gliding, they're using the reputation, but not actually working any more.
sit on one's laurels 現在の栄誉に満足して何もしない。
6) In English often "sit back" means sit down and relax, lean back, just let everything flow. If you sit up or sit straight, it means your alert to your paying attention when you're ready for work.