A 185 Other.

From the definition:
negative level

A loss of vital energy resulting from energy drain, spells, magic items, or magical effects.
It's a negative level from a spell directly, rather than from an energy drain or negative energy effect. As such, protection against those specific attacks does not block negative levels from Fell Drain.

A 187a

Yes, you can target a square you have line of effect to but simply can't see. The AC of a square is 5 (base 10, -5 for 0 DEX). If your attack is high enough to hit the square's AC you'll be on target; if there's a creature in that square and your attack is also high enough to hit their AC then there's a 50% hit/miss chance.
Total Concealment

If you have line of effect to a target but not line of sight he is considered to have total concealment from you. You can’t attack an opponent that has total concealment, though you can attack into a square that you think he occupies. A successful attack into a square occupied by an enemy with total concealment has a 50% miss chance (instead of the normal 20% miss chance for an opponent with concealment).
If your attack is not high enough to hit AC 5 then you'll miss the square.

A 187b

You'll have to ask your DM in this case. There are rules for where your shot goes with a missed thrown weapon; it's a 1d8 roll to choose adjacent squares (see page 158 of Player's Handbook), and your DM could adopt that mechanism as a house rule for projectile-driven ammunition. Note the following, though:
Generally speaking, ammunition that hits its target is destroyed or rendered useless, while normal ammunition that misses has a 50% chance of being destroyed or lost.
A damaged magic item continues to function, but if it is destroyed, all its magical power is lost.
An Illuminating (Magic Item Compendium, page 36) arrow which is destroyed on impact can't illuminate anything.