Can I play Guzma if my opponent doesn't have any Pokemon on their Bench?
No, you cannot. If the opponent doesn't have a bench there's no way to do the second part of the trainer, and you cannot play a Trainer card for no effect.
Source: TPCi Rules Team (2017-08-31)
If I use Guzma and choose my opponent's Primal Groudon-EX that has the "Omega Barrier" Ancient Trait, do I still get to switch my own Pokemon?
Since Omega Barrier prevents the first effect of Guzma, you do not get to switch your Pokemon.
Source: TPCi Rules Team (2017-08-31)
If I play "Guzma", who chooses which Pokemon to switch for each player?
You make all of those choices.
Source: Burning Shadows FAQ (2017-07-20), TPCi Rules Team (2017-08-03)