Hi Tony,
Every guitar builder will have their own way of shielding and some will even leave it out completely. I'm no expert, but I can help with some of your questions.
* The bridge must have a connection to earth. Without it your guitar will pick up everything, including truck CB radio.
* As long as everything that needs to be earthed is connected somehow to the tab on the guitar jack, it should be fine. The pots are the easiest place to connect to. You don't need to connect the cavity shielding to the bridge if they are both already connected to an earth elsewhere.
* If you are applying shielding to the back of the scratch plate, its neater/easier to remove the pots, apply the foil, then put the pots in. It also creates a physical earth connection between this shielding and the body of the pots.
* You don't need to apply any shielding (but it will sound better if you do). Its up to you where you want to apply it. I like shielding the cavity behind the pots, the pickup cavities and the back of the scratch plate.
* Earth cables won't act like antennas (unless you get some wiring seriously wrong) but if you can find another closer earth point to join to, that should be fine. I like to join all my cavity foil together and only have one wire joining this to the back of a pot, less wires = neater and easier to fault find if needs be.
Lastly: the springs hold the pickups in a sort of floating tension, probably to stop body vibrations from affecting the pickups sound. Foam under the pickups would probably change the sound for the worse. At this stage, the best advice I can give is try it out, see what you think, you can always change it later.

... or what Dingo said.