Here's a very simple answer to your #1
http://www.b737.org.uk/hydraulics.htm
The using units return the fluid to the resivoir from whence it came. B to B and Standby to Standby. (Okay spoilers, brakes, reversers are exceptions - quit hollering, I'm simplifying)
The standby resivoir is serviced by the B system resivoir at the 72% level but it is a stand-alone system when used and the fluid doesn' t intermingle with the other systems. (Please see parenthetical statement above)
The leading edge devices, when actuated by the standby system are a relatively low drag device without the penalties in speed that the trailing edge flaps cause. If they are extended by the standby system you're looking for a place to land because things have really gotten bad. You won't be needing to retract them in the near future.