Not sure how feasible this is but, It would be great if there was a filter or constraint within Marketo which could tell you if a Lead viewed the HTML or Text version of an email.
Does Marketo infer when a text email is opened? For example, if someone clicks a link within a text email and you know they didn't open the HTML, does the system show them as having opened an email (or does their history say "Sent Email, Clicked Link" with no "opened email"? If Marketo isn't currently doing this, it could be a way to derive potential text emails (or viewed through preview pane).
The activity log would only show "Sent Email, Delivered Email, Clicked Link" as you described, but we have business logic on top of our Email Performance Report that would also increment the open count. See this reference: Why Does My Smart List Count Not Match the Email Performance Report?
So, maybe you could attempt to use that as a proxy, but I wouldn't recommend it as there is also the case where the user had "always show images disabled" in their email client and then clicked the link in the HTML email...this would produce the same result as the scenario you outlined.
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