Legal counsel is a common exception to NDAs. Different jurisdictions have different specific rules. And most people would have tried to hide it. But saying you can't do that isn't true categorically.

She could have taken files that weren't material. But that's just speculation.

Google claimed the documents went to multiple external accounts. So they must know which accounts. So Mitchell could say who owned them. And they could verify.

Any lawyer will tell you to preserve evidence yourself if you can.

How would people sympathetic to her cause preserve evidence without violating Google's policies?

What might be disclosed during the trial is the same as if Google provided the files.