In a single-elimination match that goes to time and is tied during the final game, if I add prizes to my opponent's field with Nihilego GX's "Symbiont GX" attack on my turn, do I win?
Yes. After regulation time (plus any additional turns) is completed, once you have fewer prizes remaining than your opponent, you win.
Source: TPCi Rules Team (2017-11-02)
In a Sudden Death game, if I add prizes to my opponent's field with Nihilego GX's "Symbiont GX" attack, is the game won by the next player to take a prize, or the first player to take their last prize?
A Sudden Death match is complete when a player has taken all of their prizes, regardless of whether there is more than one or not. So adding prizes in this scenario makes it more difficult for the opponent to win.
Source: TPCi Rules Team (2017-11-02)
When using Nihilego GX's "Symbiont GX" attack, can you place more prizes than your opponent started the game with?
Yes, you can. The prize pile is not limited to any maximum amount.
Source: TPCi Rules Team (2017-11-02)