Following the order of execution, I think these 2 validations can be simplified a lot:
The last output of 'QuantumCircuit' will be the last register to be initialized. Therefore, we can just pass a variable to the last output of 'QuantumCircuit' that tells the 'register' node to initialize qc .
The second validation is not even required anymore, the qubits objects will always be evaluated after the register nodes & qc are all initialized.
Enhancement Description
The following statements references to @Theo-Reignier