Rulings

The creature that died doesn't have to still be in its owner's graveyard to satisfy Bontu's combat restriction.
Creature tokens that die are put into your graveyard as normal (and cease to exist soon after). If one died this turn, it satisfies Bontu's combat restriction.
All attackers are chosen at once. You can't attack with one, sacrifice it to satisfy Bontu's combat restriction, and then attack with Bontu.
In a Two-Headed Giant game, Bontu's activated ability causes the opposing team to lose 2 life and you gain 1 life.