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The ivy will not overtake it right away, but you will need to make sure that you regularly pull the ivy away from the camellia, or it will overtake it eventually. Camellias do not spread like ivy. They are more of a shrub like plant.

Generally, cutting off more than a third of the plant is not good. However, as long as the stems are still alive, it should regrow. For future reference, this article should help: https://www.gardeningknowhow.com/ornamental/flowers/camellia/pruning-camellias-how-to-prune-a-camellia-plant.htm

It may be suffering from shock. Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm

I would not cut it back by more than a third, as this could be detrimental to the plant. This article will help with pruning camellias: https://www.gardeningknowhow.com/ornamental/flowers/camellia/pruning-camellias-how-to-prune-a-camellia-plant.htm

It sounds like a camilla or a gardenia.

If the stems are still alive, you can recover the plant. This article will help you:
https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm