well I can see that none of you remember 2e. This 3e/M:tG (I love that game) way of thinking has no place in 4e. I am almost GLAD that the rules are so ambiguous and probably wrong. They simply require a little common sense to fix. Here is how I am going to play:
A "usage" is the combination of: a character, a power, a weapon/implement, and an action (standard, move, or minor).
A usage has whatever keywords make sense. If you have a flaming sword that is on the usage has the fire keyword, if the fire is off then obviously it doesn't. If a power deals fire damage then it has the fire keyword. If a power has the fire keyword then it deals (at least some) fire damage.
If the power deals radiant damage through a weapon, and you use it with a flaming weapon that is off, the damage is radiant. If you use it with flaming weapon that is on then the damage is both.
If the power deals radiant damage and you use it with a weapon that deals extra fire damage on a crit, then on a crit the base damage is radiant, the bonus damage is fire, and the usage has both keywords. If there is no crit then the damage is radiant and the only keyword on the usage is radiant.
Common sense guys, not rules lawyers. 3e brought us back to 1e, 4e brings us back to 2e