Presenting a lower C-card is more likely to get you out of babysitting duty than anything else. It will have no bearing on whether you're held to a higher or lower standard of care in the event you choose to intervene.
No matter what certification level you disclose, any lawyer with half a brain will subpoena your certification records from your certification agency. Once the lawyer gets your cert history, he'll discover what he'll play up as a major "lie"--you said you're only AOW when in fact you're a DM--and he'll describe to the jury in loving detail how much extra training you have, and how you should have known X or Y or whatever, and how you completely botched the rescue, and that you lied about your certification level to attempt to escape liability. THAT is how your higher cert level will be "held against you."
You have to understand that lawsuits have very little to do with "THE TRUTH." I tell clients that "the truth" is merely an interpretation of evidence that one side convinces a judge or jury to adopt. Building (and trying) a case is developing and telling a convincing story woven of evidence that supports a particular position, while showing that a different position is a less accurate story.
The *best* way to protect yourself if you choose to render aid is to have a good personal umbrella insurance policy. There's nothing like that warm fuzzy feeling you get from knowing that your tail is covered.
Like I said, "He who is best documented - wins.", and who ever has the most insurance is the best candidate to be sued.