A:
Unfortunately, none of the questions you ask are addressed in RONR.
So, if I may, I'll just offer personal opinion.
Not to be trite, but, mistakes happen.
How we deal with them is often a measure of our honesty and integrity.
That your Board has admitted its mistake and wishes to make it up to the excluded members says a lot.
I recommend that the minutes simply state what happened, much as the preface to your question, above, does.
They could state that an executive session was held, even though the content of the session turned out to be non-confidential.
Because no confidential issues were discussed in the executive session, I believe (again, my opinion, not RONR) the board could order the minutes be made open.
What else can be done for them besides apologize?
That's another tough question.
Perhaps the Board's official apology could be ordered entered in the minutes.
And maybe everyone could just go out for a beer.