The fire from the matches consumes the oxygen inside the tennis ball (which is why it must be sealed). After enough oxygen has been depleted, it will explode.
No. There is no pressure difference inside and outside the balloon. Popping the balloon will create some local turbulence, sending the confetti somewhat outside the original shape of the balloon,...
You could call them, and say that you messed up but are really sorry and want to fix it, you just need the right materials. They'll likely appreciate your honesty...