I like option 1 the best, because I play the North American rules. They specifically clarify your question on p.13 of the Instruction Booklet under the
Hero Movement section:
...You may
not share a square with another Hero
or monster. Exceptions: When
on the stairs or in pit traps, sharing a
square is permitted.
Of course, the European rules don't have this clarification, so play it another way if you like. Consider, however, the European rules text:
"When moving, characters and monsters may not:
move onto an occupied square. Characters and monsters may, however,
pass through an occupied square, provided that the player controlling
the obstructing miniature allows you to pass; otherwise you must take
another route or stop. Only one minature can occupy a square.
I feel the keyword is
move. If a character jumps a pit trap, he must have enough movement to get across, then jumps. I don't think jumping is actually moving onto a square, though it is a special kind of movement. The unintended consequence of falling onto an occupied pit trap square isn't exactly moving onto an occupied square either, because the character never actually moved onto the square, he instead fell. Since the Elf in your example fell (rather than moved) onto the occupied pit trap square, he should be allowed to share the square with the Barbarian.
I know this is interpretive, but to bolster the point consider the restriction that a character may only pass (read move) through an occupied square provided that the player controlling the obstructing miniature allows it. That restriction shouldn't hold if the 'obstructing' miniature is now below the level of the character that wants to pass. Therefore passing through (jumping) a pit trap sqaure is again a special case of movement, not bounded by the rule of moving onto an occupied square. Illustrating again using your case above, it isn't feasible that the Barbarian, who fell into the pit trap, could prevent the Elf from attempting to jump across the same pit. Nor could he prevent the Elf from falling in with him.
If you still feel a pit trap must be too small for two or more characters and should take precedence, I'd say go with choice of B). That way a failure still results in a Body Point of damage, and his turn ends anyway. As he just unsuccesfully jumped and tumbled, he should also defend with the -1 penalty until his next turn, just as if he was fighting from a pit trap. The rules are satisfied, you have only one character in the pit trap, and you merely moved the Elf adjacent, which still allows the monster to close and attack.
