Would it stay in its Unbound Form even if the other person didn't have the prison bottle or would it revert back to its Confined Form?
Hoopa Unbound will revert into Hoopa Confined if three days have passed after using the Prison Bottle or if it is deposited in the PC.
This snippet taken from here does not mention anything to do with its form reverting when traded . So it most likely won't (as there is no way of checking for sure this is all I can give).
Hope I helped :)
hoopa is not obtainable in a legit way yet, so you can't trade it. GTS will know when it is hacked. So it is not possible to know at this time.