…Most of the players didn’t have a hand, so they discarded one of their cards in play. Meanwhile, I had a hand of one card that I didn’t want to lose, so I discarded a card of mine in play as well. Can we do that?
A: Unless a card specifies that you discard a Keeper (or Creeper) from in play the default is from your hand.
What should have happened is that, for players with no cards in hand, nothing happens – they have nothing to discard. It’s quite common for a card to be played that simply has no effect at that specific time… and of course, you, with one card in your hand, were actually required to discard that card. Sorry.