1.) Read magic jar carefully. This spell doesn't get a lot of attention and is often run completely wrong in 3.5 games.
You don't jump into the magic jar and then target a particular creature. You leap into the jar, sense the relative power of nearby creatures and then select a power level of creature to attack. You'll then go after a random creature in that power level.
YOU CAN NOT DETERMINE THE *TYPE* OR *POSITION* OF THE CREATURES YOU SENSE FROM INSIDE A MAGIC JAR. You can sense a power level difference of 4 HD or more, so you may be able to figure out an exact ordering of power levels via a slow process of comparison, but that would take awhile ... (IE; A is 4 HD or more greater than B, C is within 4 of A and B, D is within 4 of A and C, D is also more than 4 greater than B, thus in diminishing order of HD, the order is D, A, C, B. If this really comes up, I'd suggest the DM have the player figure out the word problem and then allow the PC to figure it out in a similar time, modifier byt he intelligence modifier of the PC relative to the intelligence modifier estimated for the player.
You sense all creatures in range of the spell, which grows with your caster level. A 20th level caster can use magic jar to sense all life forces within 200' - and may end up with hundreds of creatures from which to choose to possess. In these cases, I strongly suggest the DM only provide info on a subset selected by the player (most powerful, those in the 8 HD range, etc ...) to save time.
The spell attacks life forces, so it is useless when there is no lifeforce to attack, as is the case with constructs or mindless undead.
This is a hard spell to use in a dungeon setting. My PCs tend to use it before entering a dungeon, to gaint he body of a hill giant or other beast wandering the hillsides with fewer other creatures to get confused amongst.
2.) They are seen, as already mentioned.
3.) There are no official stats for these types of explosions, so you'l need to make them up.
There are two options: Real world explosions or cinematic/fantasy explosions.
A real world explosion would be difficult to calculate. We don't know the purity of the explosive, or the first issue. It could be a rather weak thing or a rather condensed version ...
Even if it is not realistic, I might go with the fantasy version of an explosion, using a metoer swarm effect as a basis. Everyone in a X' radius takes Yd6 damage (save for half) except the one carrying the barrel who gets no save and takes a little extra damage (2d6) from the force of the blast. For a barrel of explosives, I might go with 10d6 in a 20' radius. This would be separate frm the fireball.
Also remember that the fireball would need to penetrate the barrel to cause an explosion. Attended objects generally take no damage from a spell like fireball, but a DM could overlook this rule for something as large as a barrel. The barrel would get the wizard's reflex save for 1/2 damage. Wood has a hardness of 5 and 10 hit points per inch of thickness. Fire does 1/2 damage to objects. If the barrel's wood were 1/2" thick, the fire would need to do 20 points of damage to it to pierce it. If it were an inch thick, the fireball would need to do 30 points of damage.
