Could bitcoin "survive" a definitive hard fork, spawning a new currency? What happens if two different groups have a definitive disagreement about the evolution of the bitcoin protocol? I have read is several places that the "majority" somehow would win, but is this really true, or could two different branches (A and B) of the blockchain (now tree) survive indefinitely in parallel?

I imagine that since the coins can be spent twice, once in each branch, they would somehow have their effective value halved. Suppose I'm a supporter of branch B, I would want to sell the A versions of my coins to buy more B-coins. The exact exchange ratio between the two new type of coins would depends on the relative size of the two communities. Is this scenario plausible, or is it economically unstable?
You've described it very accurately. When there's a protocol change that not everyone adopts, you'll have essentially two different currencies, with any coin in the original currency turning into a coin in each of the new block chains. The total value of the coins in both chains will be roughly equal to the total value of the coins in the original currency. The division of value between the two currencies will depend on the respective demand.

However, if both currencies use the name "Bitcoin" to describe themselves, it will be very confusing, and will make both less usable because it's hard to know what someone means when he says "Bitcoin".