It is not a voluntary agreement that the net profit from the Crown Estate goes to the Government, it is following a number of Acts of Parliament, the most recent being in 2011.
It IS a voluntary agreement. It might be ‘voluntary’ in the sense that the Queen ‘voluntarily’ asks the leader of the party who wins an election to form a government but it’s still voluntary because it’s something each individual monarch agrees to when they ascend the throne. Technically the Queen has the right to command the armed forces but she ‘voluntarily’ doesn’t do that either. Legislation refers to how it’s run and how the army is run, but that doesn’t mean the legislation compels the Crown to do something (because for a start it wouldn’t normally be legally enforceable because you can’t take legal action against the Crown).
Also the ownership isn't complicated at all. The property belongs to The Crown, so whoever is the monarch of the day. The responsibility for running it is devolved to The Crown Estate Board and the net profits go to the Government. It is that straightforward!
It’s that straightforward at the moment because we have a monarchy. If the monarchy is abolished it’s not straightforward at all. IT CAN’T BELONG TO THE CROWN IF IT DOESN’T EXIST!
There is an awful lot of anachronistic legislation surrounding the Crown and the Royal Family which is still on the statute books despite technically actually being illegal, particularly because of the Human Rights Act.
A particular example of this is the Royal Marriages Act. It barred members of the Royal Family from marrying in register offices. But that was actually illegal under the human rights act as was confirmed to allow Charles and Camilla to marry in one.
We still have laws in place which are illegal under the human rights act and we know are illegal under the human rights act but we still keep them. Like the first six people in line to the throne needing the Queens permission. Completely illegal. We know it’s illegal. Yet it’s still one of our laws despite us being completely aware it’s a totally illegal law.
The same applies to the Crown Estates and laws in the Human Rights Act about rights to property and protection of property from government interference. If the monarchy is abolished and the Crown ceases to exist there is no automatic right for the state to expect it to be passed to them. In fact, the ex-monarch would have a very powerful argument that as the Crown had ceased to exist ownership of the Crown Estates belonged to them under human rights legislation because the state wouldn’t have the right to separate ‘the Crown’ from the individual because the human rights act doesn’t accept that the institution of the Crown creates legal exceptions to human rights.
We just don’t know what would happen and the human rights act gives the Royal Family a very strong position from which to argue for personal ownership.