1) If an opponent has no unit in the reserve can you steal a unit off the board?
2) Do the stolen units immediately go into play on the board or do they go into Seth's reserve?
directly on the board
3) Do the units belong to Seth for the rest of the game? (When they are destroyed in combat they go into Seth's reserve and Seth can buy them back for 1 prayer per unit)
Yes, they belong to Seth for the rest of the game