Security clearances are granted by the president, or someone delegated by him. The president has absolute authority to bypass, modify, or shut down the clearance credentialing system. There is no law or Constitutional requirement dictating security requirements or how they are applied.
As the sibling comment pointed out, this is not to say that doing so is a good idea. But it's very probably legal.
Nope, it's not illegal at all. This where one of the "traditions" should have come in and congress/the people should have burned Trump at stake for doing so, though. All those concerns about Hilary Emails 9 years ago, but we let Trump fast track his circus in no problems.
As the sibling comment pointed out, this is not to say that doing so is a good idea. But it's very probably legal.