Those are all really excellent questions but unfortunately I don't know of any empirical studies which attempt to answer them. So, my subjective impressions...
l. I would think they mean
actively using two languages. I also think it depends on when the languages are learned. If brain plasticity decreases with age the greatest effect would be at younger ages, although it would still change brain wiring at any age.
2. There might be a law of diminishing returns if you keep on adding languages.
3. My hunch is that it increases verbal IQ. Words and their subtle distinctions in meaning become so important. Whether it increases non-verbal IQ I'm less sure about but I would think that it does. I do agree that the greatest benefits probably come from speaking languages with different grammar and syntax, not from speaking, say, French, Italian and Spanish, which I can tell you doesn't "stretch" your abilities as much.
I used to think that knowing more than one language would make people more open minded because of travel and also perhaps because of brain plasticity, but I'm not so sure I believe that any more. A great many people know English nowadays, but I don't see that it necessarily makes them more open to other people or other points of view. Their prejudices and initial cultural conditioning are still very obvious to me. Perhaps it depends on the underlying personality of the people involved, their reasons for learning the language, perhaps even the age at which they learn the language. Or perhaps it requires living in a country, a total immersion in a culture. Language is so grounded in culture, indeed the "personality" of a people, that I wonder if someone learning a new language at thirty, for example, and only spending part of their life in a new country can ever be really "bilingual". Even then, with people born in the U.S. and learning English at least by five when they start school certain kinds of family conditioning trumps everything else.
Pronunciation is also affected by the ability to distinguish subtle differences in sound and rhythm and being able to reproduce them with your mouth and vocal cords. It's like a composer who can "hear" notes perfectly but can't "sing" them or reproduce them vocally. Unless you're a very gifted mimic, which most people aren't, after a certain age you just don't get rid of your accent. After all, how many Meryl Streeps are there?
Even with actors who are native English speakers, like Australians or Brits, for example, their "American" accent will sometimes falter in the middle of a speech. I always use Henry Kissinger as an example. He's a brilliant man with a brilliant command of the English language but he's very hard to understand because of his still incredibly strong German accent. He obviously just doesn't have an "ear", because he really wasn't all that old, only fourteen when he came.
That takes me all the way back to what it really means to be bilingual. If you're translating in your head I don't think you're bilingual. Maybe it's related to whether you dream in a language? I don't know. I dream in both Italian and English depending on the setting and people in the dream, but even for me I don't start dreaming mostly in Italian until I've been back for a week or two.
Slightly off topic, I have wondered once or twice whether, if dementia ever does strike, I'll be one of those people who reverts back to the first, mother language, in my case Italian, and most people surrounding me won't have a clue what I'm saying! Thank goodness I made my children learn Italian, even if they're not really bilingual.
@Bicicleur,
I do remember studies showing that playing chess, doing crosswords etc. has a good effect on the brain. Learning to play an instrument is very beneficial as well. In that regard it's particularly true of the piano if I remember correctly because of the fact that the left and right hand have to play different notes at different speeds and rhythm, etc.. Of course, with some musical instruments other factors come into play. You have to "hear" the notes more when playing the trumpet or violin than when playing the piano, for example.