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1a: That error you get when attempting to check the "audited" box on the Link field is normal and expected.
1b: That should be the way to get a relationship audited. I'm not sure why you aren't seeing it in the audit history, you might log a ticket with support about this with details of the setup.
2: Audit records have no means of "rolling up" from child objects. Your best best it to have a trigger push this info up into the parent object any time it's updated and then have that field on the parent object audited.
I hope this helps!
1b: We will open a Support ticket
2: thanks for your advice. I'll try to implement this