1) - “No thanks" / "a coke please“ is usually fine. “I don’t drink alcohol” if pressed. But my impression is that the delivery is most important (stating it firmly without looking embarrassed seems to work best)

-With people you meet regularly you seem to get less resistance being an official teetotaler than an occasional drinker who doesn’t feel like drinking one particular evening.

-Drinking very slowly can limit your intake to a single glass per evening

-If drinks are free and things are busy it’s sometimes easier to accept a glass and not actually drink it (or at least not finish it)

-Designated driver is culture specific. My impression is that in some places (e.g., UK, Scandinavia) it is expected to drink nothing if driving, while in others (e.g., France) you are often still expected to make a "small effort".

2) Quite culture specific. Fairly standard is to wait for everyone to have a drink in their hand before starting.

3) -People like to drink because social interactions often feel smoother and more natural when participants are a little drunk, and they can also be more fun

-People expect others to drink because:

 -It’s better if all parties contribute to the enhanced socializing
-Sober vs. sober and drunk vs. drunk conversations tend to work well. Sober vs. drunk ones do not (and it’s much less painful to be on the drunk side)
-Some perceive not drinking as a signal that you don’t want to or can’t enjoy yourself
-Some people see non-drinkers as implicitly judging those who do drink