just a quick question--This is my first seasonal festival, so I'm not really sure about this. If the festival ends before I have enough items to create a themed pet, will the items I already used be returned to my inventory, lost, or saved on the seeking page until next year?

Whatever is remaining from the seeking will not be returned or reusable the next time (since the admins may not be reusing the feature/pets the next time). Usually the next festival, you'll be refunded a bit of gold, but that doesn't always happen. So it's best to use as much as you can, or sell/trade with other players before the festival is over.