Sip cautiously

Depending on what kind it is it may be less effective, but I doubt they come with expiration dates on them.

Both. Because magic and fuck rules

Yes.

Less poisonous but more deadly

One of the guys who helped assassinate Archduke Ferdinand took expired cyanide and lived, so less poisonous I guess.

It depends on whether it's chemical or biological based. Some types of chemical poisons will break down into even more toxic substances while biological ones will become less poisonous or something like that. I don't remember I looked it up a long time ago