A) The search button is your friend. There are a bunch of threads on each one of those topics.
B) If you cut the back pane to be the sides, are you replacing the back pane with new glass? Seems expensive to add a couple inches of depth to the tank.
C) Re-Siliconeing your tank isn't hard, its tedious and time consuming. Use lots of razor blades to remove old silicone, use tape to make clean lines, remove the tape while the silicone is wet. Use your finger or a plastic spoon to create the beads. Make the beads about as wide as the glass is thick.
D) You can order trim from your LFS
E) I like black silicone, its available at big-box home stores. There is debate about weather the anti-mildew/mold 100% silicone is bad for fish. I've not seen any proof of it harming fish, but if you can find without its always better safe than sorry.
F) You could always replace the black plastic rim with eurobracing, it would look almost rimless.