1. It will cause more database queries, and if your system struggles to keep up then having the stats update every hand may cause problems. However, if you don't have them updating every hand then this kind of workaround stat will not work as it needs info from the last hand.
2. Refresh Rates are in numbers of hands and the expression is in seconds - they are completely separate.
3. You need refreshes every hand. The interval configuration depends on your system - Kraada explains the logic behind this in his
How To: Advanced HUD Configuration.
4. Yes, it works, within the constraints discussed.