I think Burton should meet a higher standard for overriding the privilege- having evidence that there was institutional abuse, instead of only evidence of one corrupt agent. The court trying Connally will have access to any deliberative docs that they show an "evidentiary need" for. Given the record, I would expect the court to be generous to the defense in granting access.
If that info showed others besides Connally were corrupt ( I think one other agent has been implicated), Burton would have a case that more clearly called for overriding the Executive protections.
You're right, of course, that investigating a corrupt deliberative process requires looking at the deliberative process.
"But if the salt lose it's flavor..."