Exposure. You have developed ige antibodies to brazil nut by eating them or another tree nut with similar protein structure. You should avoid all tree nuts if you are allergic to brazil nut.
Not likely. Not likely, but possibly after a period of several years.
Unlikely. Most allergies to nuts are lifelong and occasionally quite severe. Hard to imagine a diet in which these are a critical need and cannot be eliminated. It is probably best not to challenge this and fortunately brazil nuts are infrequent in commercial products (unlike peanuts).
No. Most other tree nuts will cross react with brazil nut and cause symptoms. However, even if they did not cross react, you would still likely be exposed to brazil nut protein by contamination of the other nuts due to contact with brazil nut.