If one target becomes illegal before Seeds of Renewal resolves, you'll still return the legal target from your graveyard to your hand.

If each target is illegal as Seeds of Renewal resolves, Seeds of Renewal doesn't resolve and is put into its owner's graveyard. It's not exiled.

Causing an opponent to lose the game after you've announced that you're casting a spell with undaunted and determined its total cost won't cause you to have to pay more mana.

Effects that reduce what you pay to cast a spell don't affect its mana value.