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As to being rootbound, I would give it a month or so to adjust to its new environment before giving it the stress of a new pot.

They can take being rootbound, but this one does sound extremely rootbound to the point where it may have trouble getting enough water so it will need to be repotted after it has adjusted.

You should not wipe the leaves with oil. While it does look nice, the oil can clog the stomata in the leaves. They are the equivalent of pores in your skin. It can cause problems for the plant if they get blocked.

I would wait to see if they grow new leaves. As long as the stems and branches are still alive, they will grow new leaves.

If you want to put it in a different pot for decorative purposes, you can do that at any time. If you keep it in its current pot, check the bottom of the pot every few months for signs of it being rootbound, such as roots protruding from the drainage holes. Once you see signs of it being rootbound, it will need to be moved to a larger pot.

No, it will not kill the tree. You can cut it off.

As long as the stem is alive, it will re-leaf when it gets proper care. This article will help as well:
https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm

The leaves will regrow if you give it some time, but it will look rather bare in the meantime. You can also prune it, which will force it to grow new branches and leaves. Either way will work, it just depends on how you would like it to look while it regrows. Here is some information that will help you:
https://www.gardeningknowhow.com/houseplants/rubber-tree/prune-rubber-tree.htm