Yes, the snippets are global. Never understood why they're per account in the stock Email app.
I also have a Nexus 7 (now with Android 4.3! better taste with fewer calories... ahem....)
1 - Already answered.
2 - Not happening on any of my devices.... Now, viewing incoming messages has a full screen mode (leftmost icon, an X made of four arrows...)
3 - Already answered. You can move messages into the server's Spam folder (menu in message view mode and message list mode when one or more is selected).... The rest depends on the specific mail server (will it adjust its spam filtering rules based on the moved messages?)
4 - Yes, it saves names / addresses of those you send messages to. If you have suggestions for better wording, I'd be happy to hear them.
5 - "Purge when empty" -- remove all locally stored, previously downloaded messages from the app (for this account) if it's found to be empty (no messages) when the app checks mail. Useful to those who use AquaMail, then later use a desktop mail app that deletes all messages from the server after fetching them.
"Server delete compatibility" -- a compatibility setting for Hotmail.