Magical Questions Options

[Degausser]

Okay, so I'm playing an ork mage in an upcomming Shadowrun game, and I had some questions after perusing the main book and "Street Magic"

1) What is an "Astral Shallow" and what exactly does it do?

2) what creates background count, and is it as bad as it was in SR3 (for those of you who don't remember, SR3 background count said that any fight, no matter what, counted as Background count 2 unless you were a blood mage or followed some super-violent tradition, in which case it became a POWER SITE two.)

3)Street Magic mentions that the "Astral Space" around Wuxing has become visiable. Is this an Astral Shallow? If not, then what is it and how is it different?

4)Lastly, is there a limit to overcasting? Can a mage with rank five magic fire off a Force 15 Manaball, then take 8 physical drain (Sure, he'll need to go to the hospital, but he's going to survive. The ten security guards however. . .)