I think it was not uncommon for druids – and other mystical figures in folklore – to appear before others in magical disguise. Glamour, after all, is heavily associated with the fey, which are in turn tied to the natural world and the same sort of mythological background as druids. Further, as protectors of the natural world, druids have to assess potential “civilized” threats – appearing as a wanderer, sharing a campfire, gives you a good sense of whether the new people are going to be a problem.
Mostly, though, it just seems to play up the druids’ mystical angle. They know things, deep lore of the natural world but also about the comings and goings around them that they couldn’t possibly know. They have animals as eyes and ears but sometimes they need to see and hear for themselves. I think it is an appropriate, flavorful feature. And unlike most of the druid’s class features, I don’t think it’s overpowered.
Also, be careful about thinking too much about druids as being of or defending nature, because that was not really their historical or narrative role. The word “druid” itself means “oak-knower,” where oaks are symbols of all things ancient and deep. The druids were mystics, wise men, and priests. They drew their powers from the natural world, but much of their power was knowledge as much as it was magic (of course, at the time and in the myths, these were often the same thing). Any magic they had came from their knowledge of the nature of a world that had magic built into its very bones.
In role, they may have often been apart from society, but they were still very much human and very important to society. Their counsel was sought out in all things mystical and natural, which is to say everything that the common man did not understand. They were often highly political, kingmakers or rulers themselves as high priests. They were protectors of the old ways, which included human custom as much as it did the ancient natural world.
Hand of the Apprentice is pretty specific. It doesn't make any allowances for weapons that are long or suspended from a chain, it's one ranged attack with a melee weapon against a target within 30 feet, treated as a thrown attack but aimed with Intelligence.
The answer to both your sub-questions is no, unfortunately. Unless you're using a weapon I'm not familiar with that has a quality that actually says "When this weapon is thrown through a creature's square in order to hit a creature behind, this weapon also has a chance to hit the creature whose square it was thrown through, as if X..." then throwing a weapon through someone's square just gives the end target soft cover against the attack, it doesn't give extra attacks.
Also, I hate to be a complete downer, but note that instant weapon doesn't combine well with Hand of the Apprentice. Instant weapon specifies "If the weapon leaves your hand at any time, the spell ends at the beginning of your next turn." It's missing the all-important clause "...unless the weapon has returned to your hand." so if you fling the weapon once with Hand of the Apprentice it will vanish at the start of your next turn.
Best Answer
There are two 3.5e weapon enhancements that fit the description, both priced as a +2 equivalent. Metalline is probably the one you're thinking of, and allows the wielder to change the weapon's material to silver, cold iron, or adamantine as a standard action.
Transmuting weapons are similar, and generally a stronger option. This ability causes the weapon to alter itself upon hitting an enemy (it's an automatic process, and can't be manually activated by the wielder). On subsequent attacks against the same target, the transmuted weapon will ignore all forms of damage reduction, not just material-based DR.
I'm not aware of any Pathfinder equivalent for either ability, so they're only available if your GM is willing to allow 3.5 content.