*getting rid of the quote pyramid ftw*
First off, according to Wookiepedia the Death Star's laser, when fired at maximum planet-popping power, takes at least 24 hours to recharge. Unless the Battle of Endor took several days to conclude, it was clearly firing at significantly less power.
And would this reduced power actually split apart planets?
...And if it did, wouldn't that just make the whole thing come together, with the heavier elements and compounds (including just about all the useful ones) to end up in the middle of the new planet?
At that reduced level of power? No. But the point is that the Death Star can vary it's output from "Alderaan destruction" to "blow up a space ship" to (presumably) just about anything in between.
The problem with this logic is that "Alderaan Destruction" power also "blow
up a space ship."
The material probably would start to clump together...after thousands of years of drifting. Plenty of time to scoop up anything tasty and move on to the next thing.
Depends on how explosively you blow up the planet. If you blow it apart hard enough that billions of tons of rock can avoid billions of tons of gravity (you know what I mean), it's going to be a bit difficult to collect all the metals and such from all the scattered bits. It's really easier to just mine asteroids.
Just remember how the Moon likely formed; a large space rock ran into a young Earth and blew off a bunch of large chunks. Over thousands, if not millions of years, the debris slowly gathered together until it became the big glowing thing we can see some nights.
A more accepted (AFAIK) theory is that a fair-sized space rock struck the still-molten Earth, causing a chunk of lava and at least part of the rock to go into orbit. Kinda. Science is annoying like that--it doesn't lend itself to one-sentence descriptions.