1. Yes it is possible.
2. If you have perpetual exclusive use, I am not sure why you would do it, the process is expensive and complicated. I don’t think it affects value greatly, taking into account this is an area under your lot.
You don’t say what you are using the lower space for, but I presume you are using it within the terms of Council approval? That can affect the value, if someone is using a space in a way that has not been approved, and then tries to sell it and someone checks.
3. The process involves you purchasing the freehold to that part of the common property, which entails preparation of a plan of subdivision and reallocation of common property. This is expensive and you would bear those costs.
4. Yes it does affect unit entitlements, you would expect there to be a reallocation of unit entitlements, as you would have entitlement to a greater area. You would also have to bear the costs of that.
5. You would require the approval of the other owners by special resolution, i.e. 75%.