Is it acceptable to drink alcohol in front of senior staff during semi-official occasions?

Putting ice in good whiskey is universally unacceptable, but I think the answer may depend on which country you are in. If you are 20 in the USA, then no. Otherwise, you are an adult and you decide.

There will be people with strong feelings about what is acceptable regarding drinking, and it is impossible to satisfy them all. (I have heard reports of senior faculty expressing disapproval of a postdocs choice to only have two beers.)

When it comes to more institutional acceptance, the situation will vary. For example, when I came to Cambridge,UK, as a PhD student, there was a Wine & Cheese event to get to know the faculty members of the college. So drinking in front of them was not merely tolerated, but actively encouraged.

The safest rule is "do what others do." (But no more, and possibly less than, what they do.)

If everyone drinks, and you don't, that might be taken badly, as someone else pointed out. And if you drink and no one else does, that might be worse.

When I was younger, my rule was to drink if others did, but to have one or two less. If any one calls you on it, you can point to your "juniority." Otherwise, you're just "fitting in" without going overboard.