It's really hard to understand and I'm not sure I've got a grasp on it. But this is my best understanding.
If it's found to be illegal. Then it depends on why the judges think it's illegal.
Prorogueing Parliament is a pm right. In itself it is absolutely not illegal.
What's illegal is if he did it purely to silence parliament, to allow him to behave as a dictator, then lied to to Queen to get her approval. This would of course be illegal. That's the nuts and bolts of the court case.
If he did it primarilyfor the reasons he said to the queen, to set a new legislative agenda, then this is not illegal.
If it was illegal. Then it depends on the judges reasoning. So for example if they say it was illegal, as such it has to be recalled, because it was never prorogued legally, then of course Boris has the option to prorogue it properly, as long as his reason for doing so is legal. Ie setting a new legislative agenda.
The English courts said it was political and not a court decision. The Scottish courts said it was illegal and he lied about why he was doing it.
The Supreme Court needs to now decide. Is it a matter for the courts and do they have a position to make the pm accountable, or is it nothing to do with them. That's what's on the table
But prorogation in itself is not illegal. What's being tried as possibly illegal is why he prorogued it and that he lied to the queen to get her approval.
If he's found to have lied to get royal assent. That's a really really big deal. Enormous. And he won't be in power long. It's as corrupt as it gets.